Preheat the oven to 400°F. Wash and dry the potatoes, then pierce each potato 2 to 3 times with a knife or fork on all sides.
If desired, Combine salt, pepper, rosemary, and/or thyme in a small bowl and sprinkle all over a cutting board. Brush the potatoes with olive oil and then roll each potato on different sections on cutting board. Place the potatoes directly on the oven rack or a baking sheet and bake for 60 to 70 minutes, or until fork tender.
Remove the potatoes from the oven and let them cool until they are safe to handle.
Meanwhile, cook the bacon until crisp and crumble it. Reserve ¼ cup for top. Sauté the chopped onion in a tablespoon or two of bacon grease until softened and translucent. Add garlic and cook until fragrant. Set aside and allow to cool. Reduce the oven temperature to 350°F.
Slice each potato in half lengthwise to create 8 potato boats. Using a spoon, carefully scoop the flesh into a large mixing bowl, leaving about ¼ to ½ inch of potato around the edges so the skins remain sturdy enough to hold the filling.
Add the scooped potato, cooked bacon, sautéed onion, garlic, cream cheese, sour cream, milk or cream, remaining melted butter, salt, pepper, Parmesan cheese, and ½ cup of the shredded cheddar cheese to the bowl. Beat with a hand mixer or mash and stir until smooth and creamy. Taste and add remaining ½ cup cheddar cheese, if desired. Add a little more milk if needed to reach your desired consistency.
Arrange the potato skins in a large baking dish. Spoon or pipe the mashed potato mixture evenly into each potato shell, pressing gently to fill every corner. Sprinkle the tops with the remaining cheddar cheese and additional bacon crumbles.
Bake for 15 to 20 minutes, or until the filling is hot and the cheese has melted. If desired, switch the oven to broil for 2 to 5 minutes until the cheese is golden brown and bubbly, if desired. Let the potato boats cool for a few minutes before serving, then enjoy!