Preheat broiler with oven rack 8 in. from heat source. Cook quinoa according to package directions. While quinoa cooks, coat a large, rimmed baking sheet with nonstick cooking spray.
Toss together mushroom caps, bell peppers, onion, canola oil, chili powder, coriander, and 1 tsp. salt on prepared baking sheet; arrange in a single layer. Broil until veggies begin to char, about 5 minutes. Stir and broil again, until fully charred and tender, about 5 minutes.
Process avocado, sour cream, lime juice, cilantro, and remaining 1/4 tsp. salt in a blender until smooth.
Divide quinoa among four bowls, top with mushroom mixture, and drizzle with sauce. Top with pepitas.