Preheat the oven to 200°F. If making multiple omelets, store the cooked omelets in the warming oven until all are ready to serve.
In a non-stick pan, heat oil over medium heat. Add the peppers, onions, mushrooms and chicken sausage; cook, stirring frequently for approximately 4-5 minutes.
In a bowl, whisk the eggs with a pinch of 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. Allow the cheese to melt. Serve with toast and raspberries on the side.