Preheat the broiler. Bring a pot of salted water to a boil. Pierce each potato several times with a fork; put on a plate and microwave until tender, about 10 to 20 minutes.
Cook the bacon in a large ovenproof skillet until crisp. Drain on paper towels. Pour the fat into a bowl; wipe out and reserve the skillet.
Add the broccoli to the boiling water and cook 3 minutes, then drain.
Halve the potatoes lenthwise, then scoop the flesh into a bowl, leaving a 1/2-inch-thick shell. Place the shells cut-side down in the skillet and brush with the reserved bacon fat. Transfer the skillet to the broiler; broil until golden, about 3 minutes.
Mash the potato flesh with the butter and sour cream; season with 1 1/4 teaspoons salt and pepper to taste. Toss the broccoli, scallions, bacon and half of the cheddar. Turn the potato shells cut-side up in the skillet. Stuff with the broccoli mixture and sprinkle with the remaining 3/4 cup cheese. Broil until the cheese melts, 3 to 4 minutes. Toss the spinach with the dressing; serve with the potatoes.