The Bouclier de Brennus is the symbol of rugby supremacy in France. The mythology behind this shield-shaped trophy, affectionally known as The Plank, is that it was named after the famous Gallic warrior Brennus. About 400BC, Brennus led an army that defeated the Roman Legions and sacked Rome itself. Starting a French habit of winning in Rome that continues to this day.

While that is the romantic story, the truth is the creator of the shield was the artist and engraver Charles Brennus. In 1892 he sculpted The Plank based on a design supplied to him by the founder of the modern Olympic movement, Baron Pierre de Coubertin. De Coubertin and Brennus were members of the Union des Sociétés Françaises de Sports, which organised French rugby at the time.

Toulouse have won the Bouclier de Brennus an astonishing 21 times. Founded on deep local traditions, Stade Toulousain copied the stylised letters S and T on their club’s crest from a mosaic on the floor of the Basilica of Saint-Sernin in the city, honouring the initials of Saint Thomas.

Their colours of black and red are taken from the design of the robes worn by the Capitouls, the cloak-wearing leaders who governed Toulouse before the French Revolution.

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At the entrance of their home ground, Stade Ernest-Wallon, there is a small stone wall surrounded by a well-kept garden. Etched into the blocks of this impressively understated wall are the years when the Toulousains have claimed the Bouclier de Brennus. Being who they are, they also have a section where the years of their European Cup success are engraved.

Even for giants like Toulouse, the precious double of winning both the Champions Cup and the Top 14 is exceptionally rare. They have accomplished the feat only twice — in 1996 and 2021.

This tells us how monumentally difficult it is to triumph in both the 10-month ultra Marathon that is Top 14 and the high-pressure sprint of the Champions Cup in the same season. The fact that La Rochelle can achieve this double this weekend reveals the true world-class qualities of this team.

In French rugby, La Rochelle are seen as nouveau riche. Disrupters of the old aristocratic order. This is factually incorrect, because La Rochelle were founded in 1898. As their stadium is named after their heroic second World War club president, Marcel Deflandre, who was executed by the Nazis as a member of the French Resistance, they have, as the Americans say, real street cred in French rugby.

There is no doubt that Ronan O’Gara’s ultra-competitive style of leadership has ruffled the feathers of a few French roosters at other clubs. Nothing irritates the French more than an étranger who speaks excellent French, albeit with a Cork accent, and has the temerity to win competitions. Under O’Gara’s coaching, La Rochelle have embraced their role as provocateur. He has taken the “us against the world” and “nobody respects us” philosophy from page one of Munster’s intergenerational playbook and made it a La Rochelle anthem.

Foreign coaches in France are rarely embraced, and even more rarely do they find success. O’Gara has done both. In the Atlantic coastal town, he is revered and if he ran for mayor would be elected in a landslide.

Yet the rest of French rugby is just that, French. So at the Stade de France on Saturday we will have the strange situation of the new kids on the block, La Rochelle, as favourites and the Toulousain aristocracy, with the support of the rest of France, as underdogs.

All of which will not bother O’Gara or Les Maritimes one iota. Their foundations are based on sterner stuff than opinions.

With the best defensive record in the Top 14, the French rugby media have named the La Rochelle defensive system, “le Mur de l’Atlantique” (the Atlantic Wall). Named after the defensive installations built by the Germans in the second World War that stretched from occupied Calais in the north to the Basque country on the Spanish border and engulfing the strategic port city of La Rochelle.

In last week’s Top 14 semi-final, the relentless ferocious tackling of le Mur de l’Atlantique literally broke Bordeaux’s attack and forced it into disarray. When O’Gara worked under Scott Robertson at the Canterbury Crusaders, he learned from a master that defence wins championships.

La Rochelle’s defensive systems will have to be at their best because Toulouse have rediscovered a rich vein of form. In the combined excellence of their last few matches, the sheer brilliance of Antoine Dupont has shone like a beacon. Dupont is reaching standards of scrumhalf play not seen since the greatness of Gareth Edwards in the 1970s.

Like Edwards, Dupont is a player with the strategic mind of a chess grandmaster. In both attack and defence, there is truly nothing this player cannot do. Like trying to capture lightning in a jar, Dupont has been impossible to contain.

While the margins between these teams are tiny, if La Rochelle can be at their best for 80 minutes they should bring the Brennus to their Atlantic home for the first time in 125 years. Yet 80 minutes of excellence at this stage of the season is hard to produce. So another day of high drama awaits.

The reward to the winner is an old piece of wood that over the years has been dropped, kicked, kidnapped, kissed, driven around in fast cars and taken out dancing. It has been sprayed with more champagne than any Formula One world champion. Folklore has it that it has even been used as a post-championship-winning skateboard.

Yet the Bouclier de Brennus holds a value within French rugby that money cannot buy. To the team who earns the right to lift the ancient plank the rugby riches will last a lifetime.