Bake the Potatoes:
Preheat oven to 400°F (200°C). Pierce each potato a few times with a fork, rub with 1 tbsp olive oil, and sprinkle with salt. Bake directly on the oven rack for 45–60 minutes or until fork-tender. Let cool slightly.

Caramelize the Onions:
While potatoes bake, heat 2 tbsp olive oil and 2 tbsp butter in a large skillet over medium heat. Add sliced onions, season with salt and pepper, and cook for 25–30 minutes, stirring often, until deeply golden and caramelized. Add thyme and garlic; cook 1–2 minutes more. Pour in beef broth to deglaze the pan and simmer for 2 minutes. Remove from heat.

Prepare the Potato Filling:
Cut baked potatoes in half lengthwise and scoop out the insides into a bowl, leaving about 1/4 inch of potato to support the shell. Mash the scooped-out potato with sour cream, milk, and half of the shredded Gruyère. Season with salt and pepper to taste.

Mix and Stuff:
Fold in half of the caramelized onions into the mashed mixture. Spoon the mixture back into the potato skins. Top each with the remaining onions and a generous sprinkle of Gruyère cheese.

Bake Again:
Place stuffed potatoes on a baking sheet and return to oven for 10–15 minutes, or until cheese is melted and bubbly.

Serve:
Garnish with fresh parsley if desired, and serve warm.