Cut tomatoes in half and remove seeds, juice and spines. Place the tomatoes cut side down on paper towels so they drain.
Heat a large skillet over medium-high heat. Add the oil and the onion and fry until onion is fragrant but not fully cooked. Add ground beef and brown well. Stir in taco seasoning and diced tomatoes. Drain fat if necessary. Remove from heat.
Stir in 1/2 cup of the cheese and the cilantro, stirring until cheese has melted.
Divide ground beef mixture evenly between the prepared tomatoes. Top with remaining cheese.
Notes
Ground turkey may be substituted for the ground beef.These were made with extra large beefsteak tomatoes. If you have regular large tomatoes, you will require more than 4, depending on size.