1. Season lamb with salt and pepper. Heat a large straight-sided skillet over medium-high. Add oil, then lamb in a single layer. Cook, flipping once, until browned on both sides, 5 to 7 minutes. Transfer to a plate.

2. Reduce heat to medium. Add onions; cook, stirring a few times, until golden brown in places, 5 to 7 minutes. Add garlic, rosemary, and tomato paste; cook until fragrant, about 1 minute. Add wine; cook until mostly evaporated, about 1 minute. Stir in broth. Return lamb and any juices to pan; bring to a simmer. Partially cover, reduce heat to low, and cook until a thermometer inserted in thickest part of lamb (without touching bone) registers 130 degrees, 6 to 8 minutes. Serve over potatoes.