1. Heat oven to 250 degree F. In a large nonstick skillet over medium heat, add olive oil and saute onions until translucent, 5 minutes. Remove from heat, add the brown sugar and stir until melted, 2 minutes. Stir in tomatoes, bourbon, mustard, Worcestershire sauce, horseradish, salt, and pepper. Working in batches, transfer marinade to a food processor and blend until well mixed, about 2 minutes. Repeat with remaining marinade. Return marinade to the skillet and cook over medium heat until boiling. Reduce heat to low, cover and simmer 10 minutes.
2. Place ribs in a deep baking dish. Cover with marinade and bake, covered, until tender, 2 to 2-1/2 hours. (Increase oven temperature to 275 degree F after 1 hour of cooking.)
3. Prepare a covered grill for direct grilling. Grill ribs over low heat, uncovered, 30 minutes, brushing with marinade every 5 minutes and turning halfway through. Slice ribs individually and serve. Makes 6 to 8 servings.