Pat lamb dry and season with salt, pepper and 1 tbsp. crushed rosemary. In a large skillet or cast iron pot, heat oil over medium-high heat until hot but not smoking, and sear lamb 3 minutes. Turn lamb over and sear 2 minutes more for medium-rare. Transfer lamb to 4 plates.
Add garlic, 2 tbsp. rosemary, lemon juice, and stock to skillet and deglaze over high heat, stirring and scraping up brown bits for 2 minutes. Whisk in butter. Pour sauce over lamb, or serve on the side in small individual bowls.
Serve with roasted potatoes and a sautéed vegetable, such as asparagus or zucchini.