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.