Preheat oven to 425 degrees F. Pierce potatoes and microwave on high for about 12 minutes, then bake for 15 minutes. Slice off potato tops and scoop out pulp keeping skins intact. Mash pulp in a medium bowl.
Heat a small skillet over medium heat and add butter. When melted, add onion and saute until tender (about 5 minutes or so) 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, dividing evenly. Place on a baking sheet and bake until heated through, about 15 minutes. Sprinkle with parsley and add salt and pepper to taste.