Preheat oven to 425°F. Spread broccoli, mushrooms, and red onion on a large rimmed baking sheet. Toss with olive oil, salt, and black pepper. Roast for 25 minutes, until tender.
Meanwhile, prepare quinoa by combining quinoa with broth (or water with a pinch of salt) in a small saucepan. Bring mixture to a boil, reduce to a simmer, cover and cook until liquid is absorbed, about 12 to 15 minutes. Remove from heat and let steam, covered, 5 additional minutes; fluff with a fork.
Prepare Miso-Turmeric Vinaigrette by combining miso and vinegar in a small bowl; whisk until smooth. Add remaining dressing ingredients and mix well to combine.
Divide quinoa and veggies evenly into 3 bowls and spoon Miso-Turmeric Vinaigrette overtop. Garnish with sliced green onion and sesame seeds, if desired.
