Preheat oven to 425 degrees F. Microwave first the potatoes on high for 12 minutes, pierce them with fork or they will explode; bake for 15 minutes. When it is done, slice off the potato tops. Scoop out the pulp, and keep the shell/skin intact. Mash the pulp in a bowl.
Heat a small skillet over medium heat; add butter. Add the onion and saute until tender, for 5 minutes. Add onion, broccoli, and salad dressing to potato pulp and mix well.
Brush outside of potato skin shells with oil.
Spoon potato mixture into shells evenly. Place on a baking sheet. Bake potatoes until heated through, about 15 minutes. Sprinkle with parsley; salt and pepper to taste.