Preheat the oven to 400°F. Wash and dry the potatoes, then pierce each several times with a fork.
If desired, combine the salt, pepper, rosemary, and thyme in a small bowl. Brush the potatoes with olive oil and roll them in the seasoning mixture until evenly coated. Place the potatoes directly on the oven rack or on a baking sheet and bake for 60–70 minutes, or until fork-tender. Let cool until safe to handle, then reduce the oven temperature to 350°F.
While the potatoes bake, cook the bacon until crisp and crumble it, reserving ¼ cup for topping. Sauté the chopped onion in 1 to 2 tablespoons of bacon grease until softened. Add the garlic and cook for 30 seconds, then remove from the heat and let cool slightly.
Slice a round piece from the top of each potato. Using a spoon, carefully scoop the flesh into a large mixing bowl, leaving about ¼ to ½ inch of potato inside each skin so they stay sturdy.
Add the butter, milk or cream, ranch dressing, Parmesan cheese, 1½ cups shredded cheddar cheese, cooked bacon (except the reserved topping), sautéed onion, garlic, salt, and pepper to the potato. Beat with a hand mixer or mash until smooth and creamy. Taste and adjust the seasoning if needed.
Arrange the hollowed potato skins in a baking dish. Spoon or pipe the mashed potato mixture back into the potato shells, mounding it slightly. Sprinkle with the remaining ½ cup shredded cheddar cheese and the reserved bacon.
Bake for 15–20 minutes, or until the filling is hot and the cheese has melted. Broil for 2–5 minutes, if desired, until the tops are golden and bubbly. Garnish with fresh chives, let cool for a few minutes, and serve warm.