To make the sweet potatoes: Preheat oven to 375 degrees F. On a baking sheet that’s lined with parchment or a baking mat, place the sweet potatoes side by side. Roast in the oven until tender, about 1 hour. Allow to cool for about 10 minutes, until they’re cool enough to handle. Decrease the oven’s heat to 350 degrees F. Slice them in half and scoop out the inside of the sweet potato, leaving some sweet potato in the shell so it has structure. Pick the ugliest sweet potato half and scoop everything out completely and then discard the shell.
Run the sweet potato and caramelized onions, in batches, through a potato ricer and into a medium bowl. If you don’t have a potato ricer, you can simply use a potato masher. Add comté cheese, creme fraiche or sour cream, 1/4 teaspoon salt and pepper. Give it a good mix and give it a taste; adjust the salt accordingly (I added a pinch more salt).
If you wanna get fancy, transfer the mixture to a piping bag with the star-tip attached. Pipe the sweet potato back into the sweet potato shells. Transfer to the oven to bake for 15 to 20 minutes, until the tops are slightly browned.
To make the rosemary butter: in a small saucepan, set over medium heat, add the butter and rosemary. When the butter has melted, turn the heat off and allow the butter to steep in the rosemary.
Pour the butter atop the sweet potatoes and garnish with a handful of Italian parsley.