Heat oil over medium heat in a roasting pan or large skillet. Add the pancetta and cook until golden brown and crisp. Remove the pancetta to a plate lined with paper towels. Add the shallots to the pan and cook until soft. Add the Brussels sprouts and potatoes and toss to combine. Season with salt and pepper and roast in the oven until the vegetables are cooked through and golden brown. Remove the vegetables from the oven and stir in the butter and lemon juice. Transfer to a platter and top with the reserved pancetta.