1. Preheat oven to 400ºF. Pierce potatoes in several places with a fork; place on a baking sheet. Bake until tender, about 1 hour 15 minutes. Let cool for 15 minutes. Warm oil in a skillet over medium heat. Add mushrooms; sauté until soft and golden and liquid has evaporated, about 6 minutes. Add scallions; sauté for 1 minute. Remove from heat.
2. Carefully slice top inch off potatoes. Scoop out flesh, leaving a 1/4-inch border. Transfer flesh to a bowl and mash. Add mushroom mixture, spinach, mustard, sour cream and 1/2 cup Cheddar. Mix well and season with salt and pepper.
3. Divide mixture among potato shells; sprinkle with remaining Cheddar. Return to baking sheet; bake until heated through and golden brown on top, 25 to 30 minutes.