1 large bunch of broccoli or 1½ lb frozen chopped broccoli
½ lb grated cheese (a combination of American and sharp cheddar preferred)
2/3 cups of milk
1 Tablespoon flour
1 medium (about ¼ lb) onion, chopped
4-6 slices of wholewheat bread, torn into pieces
4 Tablespoons butter
casserole or 13x9" pan
Preheat the oven to 350° (or 325° if you're using a glass or lighter weight casserole dish) while you chop the fresh broccoli into chunks of ½ an inch to an inch in size (and don't discard the stems, please) or defrost the frozen broccoli. Steam or microwave to soften it. Brown the bread pieces and the onions in the butter in the frying pan on medium high heat. In the saucepot, mix the cheese, milk, and flour and heat until creamy. Stir in the breadbits. Transfer the broccoli to the casserole or 13x9" pan, fold in the cheese sauce, and bake uncovered for 30 minutes.
To the American ear, the name of the dish sounds likes it's missing a terminal noun, somthing like Casserole, Dish or Plate, but to the British ear, the name is perfectly normal. Witness Macaroni Cheese, never Macaroni and Cheese, and Cauliflower Cheese (and there's another serving suggestion).
A very basic recipe which nonetheless wows the uninitiated. Vary the cheeses, use different breads, add garlic or pepper, omit the onion; the variations are endless.