|Prepare the poblanos first. Light a grill or preheat the broiler. Roast the poblanos over the grill or under the broiler, turning, until blackened, 3 to 4 minutes. Transfer to a bowl, cover with plastic wrap and let cool. Peel, core and seed the poblanos then finely chop them.
Brush the portabella mushrooms with 3 tablespoons of the olive oil and season with salt and pepper. Grill or broil the mushrooms over high heat, turning, until softened, 10 to 12 minutes. Transfer to a plate, stem side down; let drain and cool.
Meanwhile, in a medium skillet, heat the remaining 1 teaspoon of olive oil. Add the onion and cook over moderate heat until softened, about 6 minutes. Add the baby spinach and cook until wilted, about 1 minute. Transfer the spinach to a sieve and press out the liquid.In a bowl, mix the spinach with the quinoa, cheese, cilantro and poblanos. Season with salt and pepper.
Preheat the oven to 325°. Season the mushroom caps with salt and pepper. Spoon the rice mixture into the mushrooms, mounding it slightly. Transfer the mushrooms to a baking dish and bake for about 20 minutes, or until the cheese is melted and the top is lightly browned. Serve warm or at room temperature, garnish with extra cilantro.