Preheat oven to 300 degrees.
Heat oil and butter in a heavy pot over medium-high heat. Brown chicken thighs on both sides, then set aside.
Cook diced onions in the pot, then add whiskey and allow it to reduce for a minute or two.
Pour in barbecue sauce, peach preserves, water, Worcestershire sauce, and garlic cloves. Stir until combined, then return chicken to the pot.
Cover and cook in the oven at 300 degrees for 1 ½ hours.
Serve over smashed potatoes and sprinkle with sliced green onions.