Crostini with Chickpeas & Tomatoes

Ingredients

• 4 tablespoons olive oil, divided plus more for the baking sheet and baguette slices
• 1 pint grape or small cherry tomatoes
• 1/4 cup chopped onion
• 1/2 teaspoon minced garlic
• 3/4 teaspoon dried thyme
• 3/4 teaspoon dried rosemary, crushed (see note)
• Generous pinch cayenne pepper
• Kosher salt and freshly ground black pepper
• 1 (15-ounce) can chickpeas, rinsed and drained well
• 1 teaspoon grated lemon zest
• 1 1/2 tablespoons lemon juice
• 18 baguette slices cut on the diagonal 1/4 inch thick
• Rosemary sprigs, for garnish