Preheat the barbecue to medium-high heat. Place the coarsely crushed peppercorns on a plate. Coat both sides of the steaks with the peppercorns. Sprinkle the steaks with a generous amount of salt. Drizzle 1 teaspoon of olive oil over each steak.
Grill the steaks oiled side down first, about 4 minutes on each side for medium-rare doneness. Remove the steaks from the barbecue and let rest for 5 minutes. Using a large sharp knife, cut the steaks across the grain into thin slices.
Place the heirloom tomatoes in a large bowl with the baby spinach and red onion. Drizzle the salad with Marie’s Balsamic Vinaigrette and toss to coat. Season the salad to taste with salt and freshly ground black pepper.
Divide the spinach between four serving plates and place the slices of steak over the top of the spinach. Surround the salad with the heirloom tomatoes and torn pieces of fresh Burrata or buffalo mozzarella cheese.