Grilled Rosemary Lamb with Juicy Tomatoes

Ingredients:
1 (3–4-lb.) boneless lamb shoulder
Kosher salt, freshly ground pepper
2 red onions, coarsely chopped
1 bunch rosemary, leaves stripped from half of sprigs (about 1 cup)
1 bunch oregano, leaves stripped from sprigs (about 1 cup)
3/4 cup red wine vinegar
1/4 cup extra-virgin olive oil
1 cup plain whole-milk Greek yogurt
1/4 cup fresh lemon juice
1 garlic clove, finely grated
5 beefsteak or large heirloom tomatoes (about 4 lb.)
Flaky sea salt
Freshly ground black pepper
3 Tbsp. fresh lemon juice, divided
1 red onion, halved, thinly sliced
Extra-virgin olive oil (for drizzling)

Instructions:
Lay lamb shoulder, cut side up, on a work surface. You will notice it’s made up of a few muscle groups, separated by thin strips of fat and gaps where the bones have been removed. Use a sharp knife to separate the shoulder into smaller pieces along these natural seams—you should end up with 5 or 6 pieces of various sizes. Transfer lamb to a glass baking dish and season generously on all sides with salt and pepper.
Pulse onions, rosemary leaves, and oregano leaves in a food processor until finely chopped; set remaining rosemary sprigs aside. Add vinegar and oil and pulse until a coarse purée forms. Season marinade with salt and pepper, then pour over lamb, turning to coat. Cover and let sit at room temperature 2–3 hours.
Mix yogurt, lemon juice, and garlic in a medium bowl. Season sauce with salt and pepper; cover and chill until ready to use.
About half an hour before you plan to start grilling, slice tomatoes into 1/2"-thick rounds and arrange on a large platter. Season with salt and pepper and drizzle with half of lemon juice. Top with onion, season with salt and pepper, and drizzle remaining lemon juice over. Arrange reserved rosemary sprigs on top; set aside.
Prepare a grill for medium heat. Without scraping off marinade, transfer larger pieces of lamb to grate and grill until underside is very well browned, about 5 minutes. Spoon some remaining marinade over lamb, turn, and continue to grill, turning every 5 minutes or any time you see a flare-up, until lamb is charred in spots and very well browned everywhere.
After large pieces have been cooking about 15 minutes, add smaller pieces to grill and follow same instructions; they take less time to cook, and by staggering the start times, all the lamb will come off the grill within a few minutes of each other. An instant-read thermometer inserted into the center of each piece should register 140°F for medium, which is ideal. Start checking smaller pieces after 7–10 minutes. The largest piece will take closer to 30 minutes. As each piece finishes, transfer to reserved platter, setting on top of rosemary. Let rest 20–30 minutes.
Transfer lamb to a cutting board and tuck rosemary sprigs off to sides of platter. Tip platter so that accumulated tomato and lamb juices pool at one end and spoon over tomatoes. Using a long sharp knife, slice lamb into very thin pieces and arrange on top of onion and tomatoes. Season with salt and drizzle with oil.
Drizzle yogurt sauce with oil and serve alongside lamb.
Lamb can be seasoned 1 day ahead; cover and chill. Sauce can be made 8 hours ahead; keep chilled.