Lay the bacon pieces on a baking tray and freeze until hard, about 30 minutes. Meanwhile, preheat the oven to 175 degrees C. Bring a large pot of salted water to boil.
Using a sharp knife, make crosswise cuts in each potato, about 1/8 inch apart, stopping about 1/4 inch from the bottom.
Add the potatoes to the boiling water and cook 4 minutes. Remove with a slotted spoon and carefully transfer to a baking tray; let cool slightly. Pat the potatoes dry, then insert 3 pieces of the frozen bacon into the cuts of each potato, spacing the bacon evenly and letting it poke out of the top. Melt 5 tablespoons butter and brush generously over the potatoes and in the cuts. Reserve any excess butter for basting. Season the potatoes with salt and pepper.
Transfer potatoes to the oven and bake until the outsides are browned and crisp, about 2 hours, basting halfway through with the melted butter.
When the potatoes are almost done, melt the remaining 3 tablespoons butter and mix with the onions, parsley and garlic. Spoon over the potatoes and roast 5 more minutes. Transfer to a platter and season with salt and pepper.