Preheat the oven to 375°F, and adjust a rack to the center.
Lightly coat the bottom of a large, oven-proof skillet with olive oil and place it over medium-low heat. (A 12-inch cast iron skillet is perfect.)
Add the onion and cook, stirring often, until soft and slightly golden, about 10 minutes.
Add the garlic and pancetta and sauté until it’s beginning to crisp, about 5 minutes. Then add the paprika and thyme, stir to blend and cook until it’s very aromatic, about 30 seconds.
Gradually stir in the brussels sprouts and turn the heat to medium-high, and add a bit more oil. Sauté, stirring often until they are tender and becoming golden along the edges, about 10 minutes.
Deglaze the skillet with the vegetable stock, using a flat-edged spatula to scrape any bits of stuck food off of the bottom of the skillet, and back into the other ingredients. Season to taste with salt and pepper. (Please note that the pancetta is salty, you you wont need too much!)
Mix in the cream and 1 1/2 cups of the Gruyère. Sprinkle the Parmesan and remaining 1/2 cup of Gruyère on top.
Place in the preheated 375°F oven and bake until it’s sizzling along the edges, about 10 minutes.(If the top isn’t as golden as you’d like, you can place the skillet under the broiler for about 30 seconds.)