Preheat oven to 300 degrees. Heat oil and butter in a heavy pot over medium-high heat. Place chicken thighs, four at a time, in the pot, skin side down. Brown both sides, then remove to a plate and repeat until all chicken is browned. Pour off half the grease, then return pan to stove. Add onions to pan and stir, cooking for 2 minutes. Pour whiskey into the pot, being very careful if you're using an open flame. Stir and scrape the bottom of the pan, allowing whiskey to reduce for a minute or two. Pour in barbecue sauce, peach preserves, water, and Worcestershire sauce. Throw in garlic cloves. Stir until combined, then return chicken to the pot, skin side up. Cover and put pot in oven. Cook for 1 1/2 hours, then remove from oven. Serve pieces of chicken over a big pile of smashed potatoes. Sprinkle sliced green onions over the top.