Preheat the oven to 425°F.
Place the cleaned and dried potatoes on a small baking sheet in a single layer. Use a fork to prick holes all over each potato. Rub with olive oil and season the potatoes liberally with salt. Bake until fork tender, about 1 hour.
Meanwhile, set a large skillet over medium high heat with 2 tablespoons of butter. Once melted, add the onion and garlic and cook until softened, about 5 minutes. Start to add the spinach a handful at a time and cook, stirring until just wilted through but not soggy. Transfer this mixture to a bowl or plate and return the skillet to the heat.
Add the remaining 4 tablespoons butter. Once melted, stir in the flour and cook for about 30 seconds. Slowly whisk in the milk and cook until thickened. Season liberally with salt, pepper, crushed red pepper flakes and nutmeg. Turn off the heat and stir in the cooked spinach from earlier and the parmesan cheese. Set aside.
Once the potatoes are done, remove from the oven and allow to cool before handling. Cut each in half, lengthwise and scoop out the flesh with a spoon and add it to a large bowl. Season the potato with salt and pepper and add in the creamed spinach. Mash everything together until well combined.
Spoon this mixture back into the potato shells and top each with a sprinkle of parmesan. Bake until golden brown and bubbly, about 8 to 10 minutes. Serve immediately and enjoy!