Prep: 20 minutes; Cook: 15 minutes; Marinate: 10 minutes.
Rinse shrimp, and pat dry. Stir together 2 tablespoons lemon juice, 1 teaspoon oil, and 1 tablespoon each dill and oregano in a medium bowl. Add the shrimp, and toss to combine. Marinate, stirring occasionally, 10 minutes.
Stir together 1/4 cup yogurt, half of garlic, 2 tablespoons lemon juice, and 1 tablespoon dill in a serving bowl. With a vegetable peeler, cut cucumbers into lengthwise ribbons, discarding the largely seeded core. Toss cucumber ribbons with yogurt mixture and pepper, to taste.
Combine the remaining 1/2 cup yogurt with remaining 2 tablespoons lemon juice, remaining 1 tablespoon dill, and remaining garlic in a small serving bowl for raita sauce. Season with pepper, to taste.
Toss tomatoes and onion with remaining 1 teaspoon olive oil in a bowl. Grill shrimp with tomatoes and onion in a lightly oiled well-seasoned cast-iron skillet or grill pan, turning once, 5 minutes or until shrimp are bright pink and cooked through, tomatoes are softened, and onion is golden brown and tender. Transfer to a plate; cover and keep warm.
Grill flatbread until golden brown and slightly crisp. Transfer to 4 serving plates; top evenly with cucumber salad, shrimp, onions, tomatoes, and romaine, if desired. Serve with raita sauce.