Heat the olive oil and butter in a large heavy pot over medium heat. Add the onions and cook, stirring frequently, just until softened, about 5 minutes.

Add the ground bison and cook without browning, breaking up the meat with a cooking spatula or spoon. Sprinkle in the grated nutmeg, salt, and pepper. Cook until no longer rare, about 5 minutes.

Add the wine and cook until evaporated, about 10 minutes.

Add the hot milk and cook over medium-low heat, stirring occasionally until absorbed, about 20 minutes.

Add the tomatoes, swish the broth in the tomato can to get out remaining purée, bring to simmer then reduce heat to low and cover the pan. Cover the pan, vent slightly, and allow the sauce to gently simmer, stirring occasionally and breaking up tomatoes with cooking spatula/spoon, until the meat is very tender, at least 1 hour but ideally 2–3 hours (or up to 6 like the chefs at Canal House!)

Add water, if needed, to keep the ragù loose and saucy.