Preheat oven to 400°F. In a large skillet, heat 2 tablespoons olive oil over medium-high heat. Add onion and garlic and season with salt and pepper. Cook, stirring often, until softened, about 5 minutes. Transfer onion mixture to a bowl.
Add chickpeas, cumin, coriander, 1/4 cup parsley, and 1 tablespoon olive oil to bowl; season with salt and pepper and toss to combine.
Arrange bell peppers in a baking dish and fill with chickpea mixture. Roast until peppers are tender, about 30 minutes.
Toss tomatoes and cucumbers with lemon juice, remaining 2 tablespoons parsley and 1 tablespoon olive oil.
Serve peppers topped with tomato-cucumber salad and drizzled with tahini.