La recette literally translates to "the recipe". It's an interesting phrase in this song because it's used metaphorically.
In the context of the lyrics, "La recette, c'est toi et moi" (The recipe, it's you and me) implies that the 'recipe' for their relationship, for happiness, or for overcoming challenges, is simply their togetherness. It's a sweet and unique way to express the essential role of their bond.
"La Recette" is Slimane pleading with the person he loves not to walk away. The title means "the recipe", and his answer to every doubt is the same one: the recipe is you and me. Nothing more complicated than that.
The verses show a home that has stopped being peaceful. Voices are raised, things get broken, and yet he keeps pointing back at how long they have been together. He is not arguing that everything is fine. He is arguing that whatever is wrong deserves a reason before it becomes an ending.
So he asks for one. Lay the cards on the table, tell me why all these fears, but say it gently, because I will understand. It is a rare thing for a break-up song to ask for an explanation rather than assign blame.
The chorus is where the song lives. "Ne me quitte pas" repeats until it stops sounding like a request and starts sounding like a reflex, the thing he says because it is the only thing left in his head. By the last verse he is back where he started: everyone around him could leave, and he would still be fine, as long as she is there.