Preheat the oven to 350 degrees F.

Combine the topping ingredients in a small bowl, mix, and set aside.

In a large skillet over medium heat, add the olive oil. After that, add the spinach in batches, and cook, stirring until wilted, about 5 minutes. Transfer to a bowl and set aside.

Next, in the same skillet add the butter, melt it, and stir it until bubbly. Add the minced garlic and flour and stir constantly with a wooden spoon, for 2 minutes, until a roux forms.

Have the milk next to you and once the flour and butter are combined, whisk in the milk and cream little by little. Season the mixture with salt, pepper, paprika, and red pepper flakes. Cook, stirring frequently, until the mixture thickens, about 5 to 7 minutes.

Reduce the heat to low and stir in the cream cheese until completely combined.

After that, add the shredded Gruyère little by little, stirring well after each addition to make sure the cheese is fully incorporated before adding more.

Stir in the parmesan cheese. The mixture will be very thick at this point.

Add the spinach with juices from the bowl back to the pan, and stir until well combined.

Add the remaining ½ cup of French fried onions and stir.

Transfer the mixture to an 8-by-8-inch baking dish.

Sprinkle with the prepared topping.

Bake in the preheated oven until the casserole is bubbling and the topping is golden brown, about 30 to 35 minutes.

Let sit for at least 10 minutes before serving. Top with freshly grated lemon zest before serving.