Preheat oven to 400°F (200°C). Pierce potatoes several times with a fork, place on a baking sheet, and bake for 50–60 minutes, until tender when pierced with a knife.
Remove potatoes from oven and let cool slightly. Slice each potato in half lengthwise. Carefully scoop out the insides into a bowl, leaving a thin shell.
In a skillet, heat olive oil and butter over medium heat. Add onion and mushrooms; cook 5–6 minutes until softened and browned. Stir in garlic and spinach, cooking until wilted. Season with salt and pepper.
Mash the scooped potato flesh with sour cream, milk, half the cheddar cheese, and the mushroom-spinach mixture until smooth and creamy.
Spoon filling back into potato shells. Place stuffed potatoes on a baking sheet.
Top with remaining cheddar, mozzarella, and Parmesan cheeses.
Bake for 15–20 minutes, until cheese is melted and bubbly.
Garnish with parsley and serve warm.