Directions:
Start the Onions: Preheat your oven to 375°F. In a large skillet over medium-high heat, melt the butter. Add the sliced onions, seasoning them with salt, pepper, and thyme. Cook, stirring occasionally, until the onions are soft and caramelized.
Deglaze: Pour in the sherry or wine, followed by the beef stock. Bring the mixture to a low boil, and let it reduce until nearly evaporated, about 5-10 minutes.
Prepare Potatoes: While the onions cook, slice the top third off each potato. Scoop the flesh into a bowl, leaving the skin intact. Mix half of the onion mixture into the scooped potato flesh.
Assemble: Refill the potato skins with the mixed potato and onion filling. Top each with the remaining caramelized onions and a generous amount of Gruyere cheese.
Bake: Place the stuffed potatoes on a baking sheet and bake until the cheese is bubbly and golden, about 15 minutes. For an extra touch of color, broil on high for 1 minute.