Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon slices on paper towels, reserving 1 tablespoon bacon grease. Crumble bacon into pieces.

Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch casserole dish with reserved bacon grease.

Place milk, eggs, seasoned salt, and black pepper in the bowl of a stand mixer fitted with a paddle attachment. Beat on medium speed until smooth and pale, about 5 minutes. Add bacon pieces, Cheddar cheese, onion, and green bell pepper; beat until combined, about 1 1/2 minutes more.

Layer potato slices in the bottom of the casserole dish; pour egg mixture over potatoes and arrange sliced mushrooms on top. Cover casserole with aluminum foil.

Bake in the preheated oven, about 35 minutes. Remove aluminum foil and bake until eggs are set, about 30 minutes more.