- Heat gas or charcoal grill to highest heat. Brush onion and each eggplant slice liberally on both sides with oil; season with salt and pepper. Grill onion until charred on all sides, turning as needed. Be more precise with eggplant: grill slices 1–2 minutes, give a quarter-turn, and grill 1–2 minutes longer. Turn over slices and repeat. Remove onion and eggplant from grill and set aside.
- Brush steak on both sides with oil and season with salt and pepper. Grill to medium rare, 130–135 degrees on a meat thermometer. Remove to a cutting board; let rest 10 minutes, then slice thinly against the grain.
- Arrange grilled eggplant and charred onion slices on a platter.
- Remove stem end from tomatoes, cut in quarters, then cut into ½-inch-wide wedges. Season with salt and pepper on both sides.
- Add tomatoes to the platter. Crumble half the feta over all. Sprinkle with half the herbs.
- Place steak slices on top. Garnish with remaining feta and herbs and serve.