In a skillet, add butter, once melted, bubbling, and slightly golden, add garlic and flour. Stirring constantly for 2 minutes. Add milk, salt and pepper using a whisk, stirring until mixture has thickened.
Reduce to low heat, add cream cheese, stir until melted, add Swiss cheese in 2 batches, until fully mixed after each batch. Stir in Parmesan cheese, until fully combined, allowing mixture to thicken as stirring.
Add sauteed spinach in skillet with cheese mixture, mix well until all combined. Add ½ cup fried onions and mix well.
Place Spinach Cheese mixture in baking dish, top and spread evenly on top with Panko and fried onion mixture and bake for 30-35 minutes, until golden brown. Remove from heat and let sit for 10 minutes and top with lemon zest and ready to serve.