Heat a large nonstick skillet coated with cooking spray over medium heat. Add onion, vinegar, thyme, and salt; cover and cook 5 minutes. Uncover and cook 20 minutes or until onion is deep golden brown, stirring occasionally. While the onion cooks, add ½ cup water, ¼ cup at a time (as needed), to keep the onion from sticking to the pan.
ASSEMBLE THE CROSTINI:
Spread 1 teaspoon of cheese over each baguette slice. Top each slice with about 1 teaspoon of the onion mixture and 1 teaspoon of jam. Sprinkle evenly with thyme leaves.