Preheat the oven to 200 degrees. (If making multiple omelets store the cooked ones on a baking sheet in the oven until all are finished)
Whisk the eggs, cajun seasoning, and a pinch of salt and pepper in a bowl.
Heat oil in a nonstick pan on medium heat. Add in the sausage and onion and saute for 3 -4 minutes until the sausage is fully cooked. In the meantime toast the bread.
Pour the eggs into the pan and add the tomato. As the eggs set around the edge of the pan, with a spatula, gently lift the egg to allow uncooked egg to flow underneath. When eggs are almost set on the surface but still look moist, add the cheese. Slip the spatula under the omelet and fold over.