In a pan, heat olive oil over medium heat. Add chopped onions and sliced jalapeños, sautéing for 2-3 minutes until soft.
Add diced tomatoes to the pan and cook for another 1-2 minutes until softened.
Whisk the eggs in a bowl with salt and pepper. Pour the eggs into the pan, swirling gently to coat the vegetables.
Once the eggs are set but still slightly runny on top, sprinkle the shredded cheese over one side of the omelette. Fold the omelette in half to melt the cheese.
Garnish with fresh cilantro and serve immediately.