Melt the butter in a large frying pan and add the red onion and garlic. Cook over a medium heat until softened – about 3-4 minutes.
Add all the mushrooms and cook over a medium heat for 10 minutes, stirring from time to time.
Meanwhile, preheat a ridged char-grill pan or heavy-based frying pan. Rub a little vegetable oil over each fillet steak, then season with black pepper. Add the steaks to the heated pan and cook for 2-3 minutes per side for medium-rare steaks. Cook for a little longer if you prefer medium or well-done steaks.
Transfer the steaks to warm plates to rest for 2-3 minutes. Meanwhile, char-grill or dry-fry the cherry tomato bunches for 2-3 minutes. Serve the steaks with the mushrooms, garnished with the cherry tomatoes.