Preheat the oven to 400° F. Repeatedly pierce each potato with a fork. Pour a small amount of olive oil on the potatoes and rub to coat. Sprinkle with salt and pepper. Place potatoes on a baking sheet and bake for 1 hour, or until they give slightly when squeezed.
While the potatoes bake, heat a medium pan over medium heat. Melt 1 tbsp butter in the pan and then add in the onions. To speed up the process, add brown sugar. If the pan starts to get dry, add a splash of water or beef broth and stir.Allow the onions to cook low and slow until caramelized, about 30 minutes.
In a separate pan, melt 4tbsp butter over medium heat. Cook until a rich brown color develops, whisking often. Remove from heat quickly. Once cooled slightly, transfer all but 1 tbsp to a bowl.
Add walnuts to the remaining brown butter and heat over medium-low. Toast the walnuts for 3-4 minutes and set aside.
Cut each potato in half and scoop out the filling into a large bowl. Mix in caramelized onions, brown butter, and walnuts until combined. Fill each potato skin with the potato mixture until full. Top with cheese.
Broil for 5 minutes or until the cheese starts to brown. Top with fresh parsley and serve!