Preheat the oven to 200 degrees. If making multiple omelets store the cooked omelets in the warming oven until all are ready to serve.
In a nonstick pan, heat oil over medium heat. Add the peppers and onions; cook stirring frequently for approximately 2 minutes. Stir in the roast beef.
In a bowl, whisk the eggs seasoned with salt and pepper. Add the eggs to the pan and cook, without stirring, until egg begins to set, approximately 1 minute.
Using a heat-resistant spatula, gently lift corner of egg; tilt pan to allow uncooked egg to flow underneath. Cook until set. Repeat the procedure until the remaining egg is cooked.
Toast the bread.
Add cheese and fold the omelet in half. Remove omelet from pan. Serve with toast and sliced peaches.