Grilled Rosemary Lamb With Juicy Tomatoes

Name: 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); ¾ cup red wine vinegar; ¼ cup extra-virgin olive oil; 1 cup plain whole-milk Greek yogurt; ¼ 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)