Preheat the oven to 450° with rack in center.
Rub prepared potatoes with oil and salt.
Bake directly on the rack for 1 hour and 15 minutes, or until tender all the way through when pierced with a skewer.
Remove potatoes from the oven, split in half and cool slightly.
Scoop out and lightly mash the potato (reserve skins); set aside.
Cut skin in strips and toss with a little oil, salt, and pepper. Roast in a 400° oven for 15 minutes, or until crispy.
While the strips are baking, make the soup:
Sauté onion in melted plant-based butter until soft, about 5 minutes.
Whisk in flour; cook 2 to 3 minutes.
Stir in broth and milk. Cook and stir until thick, 4 to 5 minutes.
Add potato and simmer for 5 minutes. Mash the potato more for a creamier consistency.
Stir in parsley and salt & pepper.
Serve soup warm and top with potato skins and sunflower seeds.