Place the butter in a small bowl. Mix in the basil, garlic and lemon zest. Place a 9" long sheet of plastic wrap on a work surface. Place the butter in a 4-inch long row down the centre of the plastic wrap. Roll the butter inside the plastic wrap, into a 1-inch thick log. Cool in the fridge until firm, about 1 hour.
Preheat an indoor or outdoor grill to medium-high. Lightly oil the grill. Season the steaks with salt and pepper. Brush the tomato slices with the 1 Tbsp. of oil. Quickly grill for 30 seconds or so on one side, and then set one tomato slice, grilled side up, on each of 4 plates. Grill the steaks to the desired doneness, allowing 3 to 4 minutes per side for rare to medium-rare. Set a steak on each piece of tomato. Top each steak with a slice of the butter and serve.