Begin by heating a large skillet over medium heat. Cook the bratwurst until they are browned on all sides, then set them aside to cool.

In the same skillet, add the mushrooms, quartered, and sauté until they become golden brown. Transfer them to a large mixing bowl.

Cut the potatoes into small cubes and brown them in the skillet with a little cooking oil. Season lightly with salt, then add them to the bowl with the mushrooms.

Slice the bratwurst into rings and cook them in the skillet until they start to brown on the edges. Combine them with the potatoes and mushrooms in the bowl.

Dice the red bell pepper and green onions into small pieces. Mix them into the bowl with the sausage and vegetables, seasoning with additional salt if needed.

Stir in the shredded cheese, then transfer the entire mixture into a baking dish. Cover with a lid and bake at 350°F for about 20 minutes.

Afterward, remove the lid and bake for an additional 5 minutes. Serve the casserole immediately and enjoy!