Preheat the oven to 450˚ F. Put the potatoes on a microwave-safe plate and microwave, turning the potatoes halfway through, until just tender, 6 to 8 minutes. Meanwhile, heat 1 tablespoon olive oil in a medium ovenproof skillet over medium-high heat. Add the onion, thyme, a pinch of salt and a few grinds of pepper. Cook, stirring occasionally, until the onion is browned, 3 to 4 minutes. Remove from the heat and stir in 2 tablespoons water.
Slice the potatoes and tomatoes into 1/4-inch-thick rounds. Gently toss together in a bowl with 2 tablespoons olive oil; season with salt and pepper. Place in overlapping layers on top of the onion (it doesn’t have to be neat). Transfer the skillet to the oven and bake until the potatoes are soft and the tomatoes are wrinkled and dry, about 25 minutes.
Meanwhile, season the steaks on both sides with salt and pepper. Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add the steaks and cook 5 to 6 minutes per side for medium rare. Remove to a cutting board and let rest, 5 minutes.
Finely chop the olives and parsley; toss with the remaining 1 tablespoon olive oil, the vinegar, orange zest and a pinch of salt. Slice the steak against the grain and divide among plates; top with the olive-parsley relish and serve with the roasted vegetables.