Pierce potatoes several times with a fork and bake at 375 for approximately 1 hour. This can be done a day ahead.
Increase heat to 400, or preheat oven to 400, if potatoes already are baked.
Add olive oil to a medium pan over low heat. Add garlic and cook 1 minute. Add onions and peppers and season with salt and pepper. Continue to cook on low to moderate heat for about 10-12 minutes, enough time to let them sweat.
While the veggies are cooking, cut potatoes in half lengthwise and carefully scoop the potato "guts" out of the and into a large bowl. You want to leave about 1/4" of potato along skin. Place skins onto a baking sheet and cook for 10 minutes, while you prepare filling.
Add salt, pepper, butter to potato and mash until smooth. When veggies are cooked, add to potato mixture and fold them gently. Remove skins from oven and carefully fill them with the potato mixture. Top with Parmesan cheese and bake for 10-12 minutes.