Line a 3 1/2- or 4-quart round slow cooker with a disposable slow cooker liner. Coat the liner with cooking spray. In a large skillet cook potatoes in hot oil over medium heat about 5 minutes or until lightly browned, stirring frequently. Add onion; cook for 2 to 3 minutes more or until onion is tender, stirring frequently (the potatoes should still be firm).
Transfer potato mixture to the prepared cooker. Sprinkle with salt and pepper. Pour eggs on top of potato mixture; stir gently to distribute potatoes evenly. Cover and cook on low-heat setting about 2 1/2 hours or until eggs are set.
Using a knife, loosen omelet from disposable liner; transfer omelet to a plate. Sprinkle with cheese. Cover with foil and let stand for 5 minutes to slightly melt cheese. Cut omelet into wedges. Top with tomato. Serve warm or at room temperature.