Preheat oven to 400°F.

Combine water, quinoa and ¼ teaspoon salt in a medium saucepan; bring to a boil over high heat. Reduce heat, cover and simmer until the water is absorbed, about 15 minutes. Remove from heat, fluff with a fork and partially cover.

Meanwhile, heat oil in a large ovenproof skillet over medium heat. Add onion; cook, stirring, until starting to soften, about 3 minutes. Add garlic; cook, stirring, until fragrant, 30 seconds. Add corn, poblano, tomato, chili powder, cumin and the remaining ¼ teaspoon salt; cook, stirring, until the vegetables have softened, about 5 minutes. Stir in beans, the cooked quinoa and ¾ cup cheese. Spread into an even layer and sprinkle with the remaining ½ cup cheese.

Bake until the casserole is hot and the cheese has melted, about 15 minutes. Serve sprinkled with cilantro and with salsa (or pico de gallo) on the side, if desired.