Bake the potatoes: Preheat the oven to 400°F (200°C). While the oven is preheating, prep the potatoes. Wash them well, scrubbing all the dirt away. Then pat them dry and poke holes in them with a fork or knife (carefully). Then, season generously with salt and pepper and rub them with oil. Place them on a baking sheet or roasting tray and bake them until tender. For 14 oz potatoes, it takes about 75 minutes. For smaller potatoes (10-12 oz), it can take about 55-65 minutes. To test if the potatoes are baked, use a knife to pierce them. If it pierces easily and the potato is tender, then it’s ready.
Cook the pancetta: While the potatoes are baking, cook the smoked pancetta. Add the diced pancetta to a pan over medium heat. Sauté it, stirring occasionally, until the pancetta is cooked (and slightly crispy) and it releases the fat in the pan (about 5-6 minutes). Remove the pancetta to a plate with a slotted spoon or spatula, leaving the fat in the pan to sauté the mushrooms in it.
Sauté the mushrooms: To the same pan (now with the fat from the pancetta) over medium-high heat, add the sliced mushrooms and a pinch of salt and pepper. Sauté, stirring occasionally, until the mushrooms are fully cooked (about 10 minutes). Add the minced garlic to the cooked mushrooms, mixing it in, and sauté for a few seconds to gently cook the garlic, and then remove the mushrooms from the heat and set them aside.
Prep the baked potatoes: Cut a small piece of the side of the baked potatoes and scoop out the middle. You can use a pairing knife, a small spoon, or a cookie scoop. Make sure to keep the shells intact (leaving enough potato on the shells to hold their shape).
Prep the filling: Mash the scooped potato with a potato masher or a potato ricer (for fluffier potatoes). Add the butter and mash again to incorporate the butter. Add the milk a bit at a time until you have the preferred consistency. Add in 3/4 cup of shredded cheddar, cooked smoked pancetta (reserve a few for garnishing the potatoes), and cooked mushrooms, and fold everything into the mashed potatoes with a spatula or spoon. Taste the mixture and add salt and pepper to taste.
Stuff the potatoes and bake: Fill the potato shells with the mashed potato mixture and add the remaining 1/4 cup of grated cheddar on top. Return to the oven at 400°F (200°C) for 15-20 minutes until the cheese is golden with some golden-brown spots on top.
Serve the potatoes: Serve the stuffed potatoes with sour cream or creme fraiche and chives.