Preheat the oven to 350 degrees F (175 degrees C).

Place frozen vegetables into a 2-quart saucepan. Add 1/4 cup water and bring to a boil over high heat. Reduce the heat to medium-high, cover, and cook for 9 to 11 minutes, stirring occasionally. Drain.

Transfer vegetables to a large bowl. Add condensed soup, 1/2 cup Cheddar, sour cream, 1/2 of the fried onions, and pepper; stir until well combined. Transfer to a deep 9-inch pie plate and smooth the top.

Bake in the preheated oven for 30 minutes. Sprinkle casserole with remaining Cheddar and fried onions. Bake until Cheddar melts, about 5 more minutes.