Scrub potatoes and prick skins with a fork; rub all over with vegetable oil. Bake in 425 F (220 C) oven for 45 to 55 minutes or until potatoes yeild to gentle pressure. Potatoes can also be microwaved.
Meanwhile in a large pot of boiling water, cook broccoli, uncovered for 2 to 3 minutes or until tender-crisp. Drain and rinse under cold water; drain well and chop coursely.
Place in a large bowl. Coursely flake tuna; add to broccoli along with celery and onions. Stir together mayonnaise, sour cream and Dijon mustard; set aside. Cut each potato in half lengthwise; carefully scoop out pulp leaving a 1/4 inch (5 mm) thick shell.
Brush skins with butter; place on baking sheet and keep warm. In a bowl, mash potato pulp; mix in broccoli mixture and 3/4 cup (175 ml) of the mayonnaise mixture. Stir in the cayenne, salt and pepper to taste.
Stuff into the potato skins, mounding the tops. Return to the oven and bake for 15 minutes or until steaming hot. Can also be microwaved. Serve with remaining mayonnaise mixture spooned over each potato.
I have not included times for the microwaves as they all vary with the wattage which affects cooking time.