1. Preheat the oven to 350°. Boil the broccoli heads in water for about 10 minutes. Drain them, and then chop them coarsely, including the stems.
2. Beat the eggs in a deep bowl, add the ricotta and cheddar cheeses and the cornstarch-milk mixture. Mix thoroughly. Add the chopped onion, salt, and pepper. Add the drained broccoli, mix again, and blend the ingredients well.
3. Butter thoroughly an ovenproof baking dish and place the broccoli mixture in it. Bake for about 30 minutes. Serve hot.
