This seemingly simple dish gains an unusual sweetness from the milk, a less-than-common braising liquid for chicken. Slideshow: More Chicken Thigh Recipes
In a medium heavy pot heat the butter over medium high heat until hot. Season the chicken with 1 1/2 teaspoons salt and 3/4 teaspoon pepper. Brown the chicken, in batches if necessary, transferring to a plate as browned.
Separate the whites and greens of the scallions and cut each into 2-inch pieces. Stir the scallion whites and lemon zest into the pot and cook until the scallions are golden, about 5 minutes. Sprinkle the flour into the pot and cook, stirring, 1 minute. Whisk in the milk and bring to a simmer. Nestle the chicken along with any accumulated juices in the milk mixture and cover the pot. Braise, simmering, until the chicken is cook through, about 35 minutes.
Remove the pot from the heat and sprinkle in the scallion greens. Let sit covered 5 minutes. Season with salt and pepper to taste, the serve.