Preheat the oven to 400 degrees F (200 degrees C). Grease a 9x13-inch baking dish with cooking spray.

Arrange bacon slices in a single layer on a baking sheet.

Cook bacon in the preheated oven until browned and crisp, 20 to 25 minutes. Drain on paper towels and transfer 1 to 2 tablespoons bacon grease to a large skillet, reserving remaining bacon grease.

Add butter to grease in the skillet. Cook and stir potatoes in hot grease-butter mixture until lightly browned, about 10 minutes; season with salt and black pepper. Transfer potatoes to the prepared baking dish.

Heat about 1 tablespoon reserved bacon grease in the same skillet over medium heat. Cook and stir mushrooms in hot grease until softened, 5 to 10 minutes. Layer mushrooms over potatoes.

Place green onions in the same skillet and remove from heat. Allow green onions to soften from the residual heat in the skillet; layer over mushrooms. Crumble bacon over green onion layer; top with 1 cup Cheddar cheese.

Blend together eggs, milk, and 1 teaspoon salt in a blender until smooth; pour over Cheddar cheese layer. Top casserole with remaining 1 cup Cheddar cheese.

Bake in the preheated oven until casserole is set and a knife inserted into the middle comes out clean, 30 to 40 minutes.