Saute the onions in 2 tablespoons (30 ml) of the butter in a large heavy saucepan over moderate heat until soft but not brown. Add the mustard, cheese, milk, salt, and pepper, and stir until the cheese is melted and the sauce is smooth.
Stir in the cooked macaroni and pour half of the mixture into a buttered 3 quart (3 L) baking dish.
Arrange the broccoli on top, and cover with the remaining macaroni mixture.
Melt the remaining butter in a small saucepan and mix with the bread crumbs.
Sprinkle the bread crumbs on top of the macaroni and cheese and bake in a preheated 375F (190C) oven for 25 to 30 minutes, until it is bubbling and brown.
1/2 cup (125 ml) chopped yellow onion
3 Tbs (45 ml) butter
2 tsp (10 ml) mustard
1 lb (500 g) Cheddar cheese, shredded
1/2 cup (125 ml) milk
Salt and freshly ground pepper to taste
1/2 lb (250 g) elbow macaroni or other small pasta shape, cooked according to the package directions
1 1/2 cups (375 ml) broccoli florets, boiled 3 minutes, drained, and coarsely chopped
1/3 cup (80 ml) bread crumbs or crushed saltine crackers