Chicken and Kale Casserole

INGREDIENTS

• Coarse salt and ground pepper
• 3/4 pound large pasta shells
• 2 tablespoons unsalted butter
• 1 large yellow onion, diced medium
• 3 garlic cloves, minced
• 2 bunches kale (1 1/2 pounds), tough stems and ribs removed, leaves coarsely chopped
• 2 cups shredded or chopped cooked chicken (from 1/2 rotisserie chicken)
• 1 container (48 ounces) part-skim ricotta
• 3 tablespoons finely grated lemon zest (from 2 lemons)
• 3/4 cup Parmesan, grated (2 1/4 ounces)