Step 1

Preheat oven to 350°F. Lightly coat a 7-by-11-inch baking dish with cooking spray.

Step 2

Heat oil in a large skillet over medium heat. Add onion and serranos; cook, stirring occasionally, until the onion is translucent and the peppers are softened, about 5 minutes. Add corn and pepper; cook, stirring occasionally, until the corn is softened, about 5 minutes. Remove from heat; stir in lime zest and lime juice.

Step 3

Transfer half of the corn mixture to a blender; add milk. Secure the lid on the blender and remove the center piece to allow steam to escape. Place a clean towel over the opening. Process on medium speed until mostly smooth, about 15 seconds. (Use caution when blending hot liquids.) Transfer to a large bowl; stir in eggs, cilantro, 3/4 cup sour cream and the remaining corn mixture. Add flour, cornmeal, sugar, baking powder, salt and 1/4 teaspoon chile powder; stir to mix well. Pour into the prepared baking dish.

Step 4

Bake until lightly browned and set, 35 to 40 minutes. Let stand at room temperature for 10 minutes.

Step 5

Combine water and the remaining 1/4 cup sour cream in a small bowl; stir until smooth. Drizzle the mixture over the casserole and sprinkle with cotija and the remaining 1/2 teaspoon chile powder. Garnish with cilantro, if desired.