Preheat the oven to 375 degrees F (190 degrees C). Grease a 9x13-inch casserole dish.
Cook and stir sausage and onion in a skillet until browned, about 5 minutes. Drain excess grease.
Arrange sweet potato fries in the casserole dish in a single layer. Layer spinach, mushrooms, and sausage mixture on top. Whisk eggs together in a bowl. Season with seasoning salt and pour into the casserole. Cover with Cheddar cheese.
Bake in the preheated oven, uncovered, until set, about 35 minutes. Let rest for at least 5 minutes before cutting and serving.
