Preheat oven to 400 degrees F (200 degrees C). Grease a 9-inch square pan.
Heat a large skillet over medium-high heat. Cook and stir sausage in the hot skillet until browned and crumbly, 5 to 7 minutes; drain and discard grease. Stir corn, mushrooms, 1/2 cup Monterey Jack cheese, and 1/2 can French-fried onions into sausage until well combined. Pour mixture into the prepared square pan.
Whisk flour, milk, eggs, oil, and chili powder together in a bowl until smooth; pour over sausage mixture.
Bake in the preheated oven for 30 minutes. Top mixture with remaining 1/2 cup Monterey Jack cheese and 1/2 can French-fried onions. Bake until cheese is melted, about 3 more minutes.