Now in the 24th year of this competition, these sides have carved up eight titles between them and sit proudly at the top of the roll of honour.
The two powerhouses haven't locked horns since 2011 and since then, their trajectories have gone in opposite directions.
Leinster have annexed the tournament three times in the intervening seven years, while the Top14 outfit have struggled to match previous glories. Indeed last year they failed to emerge from the pool stages of the Challenge Cup.
Both sides won their opening games in contrasting manners. Leo Cullen's side went to town on Wasps at the RDS with an exhibition of free-flowing rugby, while Freddie Burns' brain fart at the Rec ensured a winning start on the road for Ugo Mola's men.
The home side have been dealt a big blow with the loss of Jerome Kaino through injury, while a lot of their attacking hope will rest on the shoulders of the gifted, if unproven at this level, Romain N'tamack.
The only change in personnel for the visitors is enforced with Rob Kearney ruled out with a tightness of the quad. Joe Tomane is drafted into the side with Jordan Larmour switching to the 15 jersey.
Leinster squashed Wasps last week with a devastating second-half display but such are the standards at the province that they weren't completely content.
"I think we were quite slow with our lineout process," scrum coach John Fogarty told RTÉ Sport.
"We were slow to even get to the lineout, and when we were in there we were a bit clunky in our movement. It really is down to individual error. You'd never throw someone under the bus.
"I just think we weren't as fluid in our movements when we were in the lineout and we were slow in our decision making getting into the lineout. Those two areas rattled us a little bit and that led to errors. It led to us missing lineouts.
"It's frustrating for some of the players. For us as a unit it's something we'll have to really look at because when we go to Toulouse, that's an area they were strong on against Bath and it will be an area they'll look to get after us."
An exciting game is the expectation, but the smart money is on Leinster setting a club record 11th consecutive victory in the competition.
Leinster: Jordan Larmour; Joe Tomane, Garry Ringrose, Robbie Henshaw, James Lowe; Jonathan Sexton (c), Luke McGrath; Cian Healy, Sean Cronin, Tadhg Furlong; Devin Toner, James Ryan; Rhys Ruddock, Josh van der Flier, Jack Conan.
Replacements: James Tracy, Jack McGrath, Andrew Porter, Scott Fardy, Sean O'Brien, Nick McCarthy, Ross Byrne, Rory O'Loughlin.
Toulouse: Thomas Ramos; Cheslin Kolbe, Sofiane Guitoune, Romain Ntamack, Maxime Médard; Zack Holmes, Sébastien Bézy; Clément Castets, Julien Marchand, Charlie Faumuina; Florian Verhaeghe, Joe Tekori; Rynhardt Elstadt, Alban Placines, Selevasio Tolofua.
Replacements: Leonardo Ghiraldini, David Faimafiliotama'ita'i Ainuu, Maks Van Dyk, Pierre Gayraud, Louis Madaule, Yoann Huget, Antoine Dupont, Arthur Bonneval
Follow Toulouse v Leinster on Sunday (kick-off 3.15pm) via the live blog on RTÉ.ie/Sport and the News Now app or listen live on RTÉ Radio 1's Sunday Sport