In a nonstick pan, heat the oil over medium heat. Add the onions, mushrooms, peppers and chicken sausage; saute until the the vegetables are soft, about 3 - 4 minutes.
Add the eggs to the pan. Cook, without stirring, until egg mixture begins to set, approximately 1 minute.
Lightly toast the bagel.
Using a heat-resistant spatula, gently lift the corner of the egg and tilt the pan to allow uncooked egg to flow underneath. Cook until set. Repeat this procedure until the remaining egg is cooked.
Add the cheese and fold the omelet in half, place on the bagel, cut in half and serve with raspberries.