Comforting Tortellini Casserole

Ingredients

• 2 tablespoons extra-virgin olive oil
• 1 pound Italian sausage, hot or mild
• 1 cup yellow onion, diced finely
• 4 cups lacinato kale, roughly chopped
• 1 (24-ounce) jar marinara sauce
• 1/2 cup low-sodium chicken or vegetable broth
• 1/2 lemon, zested, plus 1 tablespoon lemon juice
• 1 teaspoon kosher salt
• 1/2 teaspoon freshly ground black pepper
• 1 (18-ounce) package refrigerated cheese tortellini
• 1 cup mozzarella, shredded
• fresh basil leaves, to taste, for garnish