Line a 13 -x 9-inch pan with aluminum foil, allowing several inches to extend over sides; coat pan with cooking spray. Peel and cut each onion into 8 wedges. Place onions in pan; drizzle evenly with olive oil. Sprinkle evenly with salt and thyme.
Bake at 450° for 15 minutes; gather up edges of foil, and seal. Bake 15 more minutes. Let stand 10 minutes. Drizzle with balsamic vinegar, sprinkle with pepper, and toss.
Note: The fat percentage appears to be high because olive oil, a healthy fat, provides most of the calories in the recipe. Onions have very few calories.