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.