Spray the insert of a 4-quart slow cooker with cooking spray.
Place the bacon and onion in a cold skillet and turn the heat to medium. Cook, stirring occasionally, until the bacon is crisp and the onions are tender. Remove from the pan with a slotted spoon and drain on paper towels.
In a large mixing bowl, whisk the eggs, milk, Bisquick, salt, pepper, and cheese together. Stir in the bacon and onion mixture. Pour into the slow cooker. Top with tomato slices and cook on low for 6-7 hours or on high for 4 hours. The quiche is done when the sides are browned, the center is set, and it has begun to pull away from the sides.