Preheat oven to 350°F.
Heat oil in a large ovenproof skillet, such as cast-iron. Add corn, green and red peppers, and onion; cook, stirring occasionally, until charred, 7 to 10 minutes. Gradually add spinach in batches; cook, stirring frequently, until wilted, 1 to 2 minutes.
Stir in chicken, enchilada sauce, and salsa until combined. Gently stir in tortilla strips. Sprinkle with cheese. Transfer to the oven and bake until bubbly, about 15 minutes.
Top the casserole with tomatoes, cilantro, and radishes.
