While preparing the dish, preheat the oven to 350°F.
Heat the olive oil in a frying pan. Sauté the beef in the pan together with the onion and chopped tomato.
Add salt and pepper to taste. Cook for approximately 15 minutes. Slice the jalapeños into two pieces. Empty the insides of the slices by discarding the seeds and the veins.
Stuff about a tablespoonful of the cooked beef in the empty jalapeño halves. Sprinkle mozzarella cheese on the surface.
Arrange the filled peppers on a baking sheet and cook in the oven for 15 minutes and wait until the cheese browns.