Cut broccoli into florets, reserving stems for another use.
Arrange cauliflower and broccoli in a steamer basket over boiling water. Cover and steam 10 minutes or until crisp-tender.
Melt butter in a Dutch oven over medium-high heat; add bell pepper and garlic, and sauté 3 to 5 minutes or until tender. Stir in broccoli, cauliflower, mustard, salt, and pepper; sprinkle with onions.
Note: To make ahead, spoon broccoli mixture into a greased 11- x 7-inch baking dish (do not sprinkle with onions); cover and chill. Bake, covered, at 350° for 20 minutes or until heated. Sprinkle with onions before serving.