Preheat your oven to 375-F degrees.
Melt the butter in a skillet large enough to hold all the ingredients.
Add the onions and cook until the onions are translucent and soft, about 5 minutes, then add the garlic and cook for an additional minute.
Turn the heat off and stir in the flour, mixing well.
Add the milk to the skillet in a slow stream, whisking well as you pour until it's all combined.
Turn the heat back onto medium and whisk to thicken the sauce. Whisk until the sauce starts to simmer, then turn off the heat.
Add the salt, pepper, paprika, and hot sauce to the skillet, mixing well.
Stir in the spinach, bell peppers, the lemon juice, all of the brie, 1/2 cup of mozzarella, the pepper Jack cheese. Mix well, adding the cream until the dip is nice and thick and creamy. Don't worry if the cheese doesn't melt just yet.
Transfer the dip to a shallow baking dish (or dishes if they're small) and top it with the remaining mozzarella, and sprinkle the Parmesan over the top.
Bake for about 10-15 minutes or until the Spinach and Brie Dip is hot and bubbly, and the top is golden brown.
Serve with your favorite dippers!