Buffalo Burger with Sweet Onion Confit

Sweet onion confit:
4 tbsp. butter
2 tbsp. canola oil
4 large onions, thinly sliced
18 shallots, peeled and sliced
48 cloves garlic, peeled
3/4 cup marsala wine
3 cups demi-glace
1/4 cup fresh thyme, chopped

Burger:
4 lb. ground buffalo
4 lb. ground lamb
4 lb. lean ground beef
2 tbsp. salt
2 tbsp. pepper

3 lb. Saint-André cheese
1 cup sweet-hot mustard
1/2 cup mayonnaise
24 large buns, toasted