Prepare outdoor grill with medium-hot coals, or heat gas grill to medium, or see Oven Method, below.
Open large foil oven bag and sprinkle flour into bag. Add half the sliced onion and garlic. Season ribs on both sides with salt and pepper. Place in bag; top with remaining onions and garlic. Fold open edge of bag over at least 1 inch to seal. Place bag on baking sheet to transfer to grill.
Place filled bag on grill. Cover grill. Cook 35 minutes. Carefully flip entire bag; grill another 35 minutes.
Using a baking sheet to support the bag, remove the bag from the grill. Open the bag carefully; the ribs should be fork-tender. Remove ribs to cutting board; let cool slightly. Discard bag with vegetables and drippings.
Meanwhile, stir together the chili sauce, brown sugar, honey mustard, cinnamon, salt, thyme and pepper. Brush evenly onto both sides of the ribs.
Return ribs to grill; grill 5 minutes, turning, to heat through and caramelize sauce. Baste with any additional sauce and serve immediately. Makes 6 servings.
Heat oven to 400°F. Place filled oven bag in oven for 35 minutes. Flip bag over; bake 35 minutes. Continue with steps 4 and 5 above. Increase oven temperature to broil. Broil ribs about 4 inches from heat for 10 minutes, turning the ribs over once.