Add onions and 3 tbsp butter into a large skillet. Saute on low-medium heat for 20-25 minutes, until onions are caramelized. Stir occasionally. In a large mixing bowl, add cooked potatoes, another 3 tbsp butter and milk.Mash gently until there are no lumps. Do not "puree." Add bacon, caramelized onions, cheese and pepper. Gently stir until combine. Taste. Add a pinch of salt if necessary. Serve.