Heat some oil in a small frying pan over medium-high heat. Add in the onion, red bell pepper and green bell peppers. Sauté until everything is cooked through, about 5 minutes. Transfer to a plate.
In a large bowl, beat together the eggs and Mexican spice blend.
In a large 12-inch (30-cm) frying pan, heat a knob of butter on medium to medium-high heat. Once the pan is hot, pour on the egg mixture. It should spread across the entire bottom of the pan. Allow it to cook about 2-3 minutes, undisturbed. The eggs should be fully set on the bottom, and starting to look set on the top.
On half of the omelette, top the onions, peppers, and chicken. Gently fold the omelette over, leaving about a 1-inch gap so you can see what is inside the omelette.
Turn the heat down to medium. Allow the omelette to cook for another 5 - 7 minutes, until the eggs look fully cooked through.
Remove from heat and sprinkle on the cheese. Top with salsa, guacamole, and sour cream. Serve immediately.