Preheat oven to 400°; bring garlic and milk to boil; drain and discard milk; in a small metal baking pan toss garlic with 1 T oil; roast 13 minutes or until golden; cool a bit and put the garlic and it's cooking oil in a food processor; process to a paste; sprinkle with salt and pepper and set aside.
Saute the onions in the butter and 1 T oil over medium-low heat; cook until golden, about 40 minutes; add vinegar, thyme and bay leaf and simmer until liquid is reduced by half, about 3 minutes; stir in cream and salt and pepper; set aside.
While the onions are sauteing, preheat the oven to 450°; heat the remaining 1 T of oil in a large skillet; salt and pepper the chicken; place skin side down in skillet and cook until golden brown on one side only, about 4 minutes.
Place chicken, skin side up, on a rimmed baking sheet and roast until internal temperature is 180°; spread garlic paste atop chicken pieces and bake another 5 minutes, until golden brown.
Rewarm onions over medium-low heat; to serve plate onions and place 2 pieces of chicken atop them; garnish with parsley.