In a medium saucepan cook broccoli and sweet pepper in a small amount of boiling, lightly salted water about 8 minutes or until broccoli is crisp-tender; drain. Cover and keep warm.
Meanwhile, for sauce, in a small saucepan melt butter over medium heat. Add onion and garlic; cook until onion is tender. Stir in cornstarch. Add orange juice and mustard. Cook and stir until mixture is thickened and bubbly. Cook and stir for 2 minutes more.
Pour the sauce over the broccoli mixture; toss gently to coat.