Pre-heat oven to 400F.
Pierce cleaned potatoes with a fork in 3-4places and, for crispy skin, rub with a teeny bit of oil. Bake on a foil lined baking sheet for approx. 40 minutes or until insides are hot and fluffy.
Heat 1 Tbsp butter in a large pan over medium-high heat.
When butter begins to bubble, add mushrooms and cook, stirring frequently, until they start to brown and soften, about 7 minutes.
Reduce heat to medium and add shallot and remaining butter to pan. Cook until shallot softens, about 1 minute.
Add broth and cook until evaporated, 1-2 minutes.
While mushrooms cook, arrange potatoes close together on a baking sheet. Cut across the top, lengthwise, and gently squeeze in sides of potatoes to puff up the middle, pulling outwards slightly to reveal more of the fluffy potato center.
Sprinkle with Feta and bake until warmed through and cheese is golden brown, about 5 minutes.
Add spinach to pan with mushrooms over medium heat and cook until wilted, about 3 minutes more. Stir in remaining heavy cream and season with 1⁄4 tsp salt and pepper as desired.
Top potatoes with mushroom and spinach sauce, a drizzle of red wine vinegar and garnish with green onions.