Step 1

Preheat oven to 400°F. Combine onion and vinegar in a bowl. Season with salt and pepper. Let stand 20 minutes.

Step 2

Coat a rimmed baking sheet with 2 tablespoons oil. Add potatoes and season with salt and pepper; arrange cut sides down. Roast 10 minutes. Add radishes to pan and roast until potatoes are golden brown and radishes are tender, 14 to 16 minutes.

Step 3

Heat 1 tablespoon oil in a large skillet over medium-high heat. Season lamb with salt and pepper. Cook to desired degree of doneness, 3 to 5 minutes per side for medium- rare; transfer to a platter.

Step 4

Add mint, feta, and remaining 2 tablespoons oil to onion mixture. Serve lamb topped with relish and vegetables alongside.