Preheat the oven to a temperature of 375°F.

In a large skillet, melt the unsalted butter over medium-high heat. Once melted, add the finely sliced onions. Season these onions with kosher salt, freshly ground black pepper, and dried thyme leaves. Allow them to cook gradually until they achieve a soft texture and adopt a caramelized color. Stir occasionally to ensure even cooking.

To the skillet, add the dry sherry or white wine, using it to deglaze the pan and incorporate the flavors from the bottom. Following this, introduce the beef broth and allow the mixture to reach a subdued boil. Continue to cook until the liquid has almost completely reduced, which should take approximately 5-10 minutes.

Concurrently, remove the top third of each baked russet potato and carefully extract the inner flesh, placing it into a medium-sized bowl. To this potato flesh, incorporate half of the previously prepared onion mixture and mix until combined.

With precision, fill each potato shell with the combined mixture and arrange them on a baking tray. Garnish the top of each potato with the remaining onion mixture, followed by a generous layer of grated Gruyere cheese.

Place the baking tray in the preheated oven and bake until the Gruyere cheese has melted and adopted a light golden hue. For additional browning, one may place the potatoes under a broiler set to high for a brief period of approximately 1 minute.

Once completed, remove from the oven and serve the potatoes while warm.