Place bourbon in a medium saucepan and set over medium heat. Gently simmer until reduced to 2/3 cup, about 5 minutes.
In another medium saucepan, heat oil over medium-high heat. Add onion and garlic; cook 5 minutes, stirring occasionally. Carefully stir in ketchup, reduced bourbon, brown sugar, vinegar, red pepper flakes, salt and pepper.
Place ribs in a large baking dish and add bourbon-ketchup sauce. Turn to coat evenly. Cover with plastic wrap and refrigerate overnight.
Heat a gas grill to medium-high or the coals in a charcoal grill to medium-hot. Lightly grease grates of grill. Add ribs meaty side up and grill, covered, 20 minutes, until lightly charred. Baste with sauce, turn and grill 20 minutes more. Baste ribs again and turn. Grill an additional 20 minutes.
Place remaining sauce in a small pot. Simmer 5 minutes.
Place racks on a cutting board and slice into individual ribs. Serve with remaining sauce on the side.