Preheat the oven to 400°F. Line a baking sheet with parchment paper or a silicone baking mat.
Carefully remove the top of the squashes and scoop out the seeds. Season the inside flesh generously with salt and pepper; transfer the pumpkins to the prepared baking sheet and set aside.
In a medium skillet, add butter and sauté onions for 3-4 minutes or until softened. Add garlic and sauté with the onions for abut 15-20 seconds, making sure not to burn.
Whisk in flour and cook about 2 minutes, stirring constantly, until the paste cooks and bubbles a bit, but don't let it brown. Slowly add cream and milk, continuing to cook and stir as the sauce thickens. Squeeze as much liquid as possible from the spinach and add spinach to the sauce (feel free to add more cream + milk for a thinner sauce). Add nutmeg and lower the heat; cook, stirring for 2 to 3 minutes more. Add the Fontina, Gruyère and ½ of the Parmesan cheese; mix well. Continue to cook a few more minutes until the cheese starts to melt. Season, to taste, with salt and pepper. Pour the creamed spinach sauce into the pumpkin. Sprinkle remaining Parmesan on top.
Put the cap in place and bake the pumpkins (depending on the size and type of your pumpkin and your oven) for 45-55 minutes or until everything inside the pumpkin is bubbling and the flesh of the pumpkin is tender enough to be pierced easily with the tip of a knife. Remove the cap during the last 15 minutes of baking.
When the pumpkin is ready, very carefully (they are hot and wobbly) transfer to a serving platter. Serve with remaining creamed spinach sauce and eat right away! Enjoy!