Preheat the oven to 400 degrees . Bake the potatoes until cooked through, about 50 minutes.
Meanwhile, in a large skillet, heat 1 tablespoon olive oil over medium-high heat. Add the mushrooms and cook, stirring occasionally, until softened, about 5 minutes. Transfer to a large bowl.
Add the remaining 1/2 tablespoon olive oil to the skillet; add the spinach and cook until wilted. Stir in the vinegar and cook for 1 minute. Add the spinach to the mushrooms; add the feta and stir until mostly melted. Stir in the pine nuts and dill; season with salt and pepper.
Cut a deep "X" into the top of each potato and squeeze together to open. Add 1/2 tablespoon butter to each potato then generously stuff each with the spinach mixture.