Arrange the pancetta in a single layer on a quarter sheet tray lined with parchment,. Lay another piece of parchment paper over the pancetta. Cook's Note: Put another quarter sheet tray on top of the parchment to fit snugly. This will prevent the pancetta from curling while baking.
Put the pancetta in the oven and bake until rendered and crisp, 15 to 18 minutes. Remove the pancetta from the oven and set aside on a paper towel lined platter.
Add the butter to a medium-sized saucepan over medium-high heat. Allow the butter to brown for 1 minute and then add garlic, leeks, and shallots. Cook the vegetables, stirring frequently until they develop a little color, about 4 to 5 minutes. Add the white wine and scrape the fonds from the bottom of the pan. Stir in the chicken stock and bring to a boil. Whisk in the milk, mashed potatoes, lemon juice, cayenne, salt, pepper and chives and stir. Keep warm.
In a deep saute pan, add enough water to fill up to within 2 inches of the edge. Bring the water to a slow simmer (a slight bubble) and add the vinegar. Crack the eggs into the water and poach until the whites are firm and the yolks are still loose, about 3 to 4 minutes. Remove from the water using a slotted spoon to a plate.
To assemble: Put 2 muffin halves on each plate and put 2 slices of crisp pancetta on each half. Top each with a poached egg and pour enough gravy over the eggs to cover and form a pool on the plate. Serve.