Recipe: Polenta Florentine

Name: Recipe: Polenta Florentine
Ingredients: Cooking spray or butter, for coating the baking dish; 2 (18-ounce) tubes prepared polenta, cut into 1/4-inch-thick rounds; Freshly ground black pepper; 1 (6-ounce) package baby spinach (about 6 loosely packed cups); 2 tablespoons unsalted butter; 1/2 medium yellow onion, diced; 1 clove garlic, minced; 1/2 teaspoon kosher salt, plus more for seasoning; 2 tablespoons all-purpose flour or cornstarch; 2 cups whole milk; 1/2 cup grated Parmesan cheese, divided; 2 teaspoons freshly squeezed lemon juice