Preheat oven to 350° F.
Cook pasta in boiling water only until al dente. Remove from heat; stir in spinach and let stand until wilted, then drain well.
Heat a sauté pan over medium heat. Add olive oil and swirl to coat.
Add onion to hot pan and cook 15 minutes or until golden brown, stirring frequently.
Add flour and minced garlic to onions; cook 1 minute, stirring constantly with a whisk.
Gradually add in milk and broth; cook 8 minutes or until sauce boils and thickens, stirring constantly.
Stir in 3/4 cup cheese, salt, pepper, and 1/4 teaspoon lemon zest (or more to taste). Remove from heat.
Combine pasta with sauce mixture, and toss gently to coat.
Spoon the pasta into a 9 x 9-inch glass or ceramic baking dish coated with cooking spray.
Sprinkle half of the Panko over the pasta, top evenly with the remaining Parmesan, then sprinkle the remaining Panko over the cheese.
Bake for 40 minutes or until browned and bubbly.
Serve with apples on the side.