Preparation

• Preheat the oven to 350°F. In a small bowl, combine ½ cup of fried onions and the panko breadcrumbs; set aside.
• Heat a large skillet or Dutch oven over medium heat, then add the olive oil. Add the spinach (in batches, if needed). Cook, stirring occasionally, until wilted, about 5 minutes. Transfer to a heat-safe bowl and set aside.
• Add the butter to the same skillet. Let it melt until bubbly and golden. Add the garlic and flour and stir constantly with a wooden spoon for 2 minutes.
• Whisk in the milk, salt, and pepper. Cook, stirring frequently, until the mixture thickens, 5 to 7 minutes.
• Reduce the heat to low and stir in the cream cheese until completely combined. Add the gruyère in 3 batches, making sure the cheese is completely incorporated before adding the next batch. Stir in the parmesan. The mixture will be very thick and will give a lot of resistance when stirring.
• Add the spinach with juices to the cheese sauce, stirring until well combined. Add the remaining ½ cup of fried onions.
• Transfer the mixture to an 8-by-8-inch baking dish. Sprinkle with the fried onion and panko mixture.
• Bake until the casserole is bubbling and the topping is golden brown, 30 to 35 minutes. Let sit for at least 10 minutes. Top with lemon zest just before serving.