Use a 4-quart slow cooker sprayed with cooking spray.
Heat the olive oil in a large skillet on the stove over medium heat. Add the mushrooms and cook, stirring, until they have browned completely and have shrunk in size. Add the mushrooms to the prepared insert.
In a large mixing bowl, whisk together the eggs, milk, baking mix, salt, and pepper until frothy. Fold in the crumbled cheese.
Pour this mixture evenly on top of the mushrooms. Cover and cook on low for 6-7 hours or on high for about 4 hours. The quiche is finished when the top has browned, the center has set, and it has begun to pull away from the sides.