To make the sauce, combine first 4 ingredients in a small bowl; stir with a whisk. Set aside.
To prepare stir-fry, bring water to a boil in a small saucepan. Stir in the quinoa; cover, reduce heat, and simmer 10 minutes. Remove from heat. Let stand, covered, 10 minutes; fluff with a fork.
Heat 1 tablespoon oil in a large nonstick skillet over medium-high heat. Add onion and garlic, and stir-fry for about 3 minutes. Add the bell pepper, mushrooms, asparagus, salt, and broccoli; stir-fry 3 minutes. Stir in the sauce. Serve over quinoa, and sprinkle with sesame seeds.