In a medium bowl, mix steak and onion until well combined; season with cracked black pepper.
Using your hands, form 4 even sized patties.
Brush patties on both sides with ½ tbs of the oil and set aside at room temperature.
In a medium bowl, combine the tomatoes, remaining oil, salt, garlic and cracked black pepper. Set aside at room temperature.
Preheat a barbecue or grill pan. Cook the patties for approximately 4 to 5 minutes on each side. Remove from the heat, cover loosely with aluminium foil and let rest for about 5 minutes. Arrange the rocket on 2 serving plates and top with the tomato salad. Serve with accompanying patties and wholegrain mustard.