Preheat oven to broil. Place 4 oven-safe bowls on a small rimmed baking sheet.
In a large Dutch oven, melt butter over medium heat. Add onion; cook, stirring occasionally, until golden brown, about 15 minutes. Add vinegar; cook until almost evaporated, about 1 minute. Add leek, garlic, thyme, salt, and pepper; cook, stirring occasionally, until tender, about 5 minutes.
Add stock; bring to a boil. Cook for 5 minutes.
Ladle soup into prepared bowls. Top each with bread cubes, and sprinkle with cheese.
Broil until golden brown, 2 to 3 minutes. Garnish with pepper and thyme, if desired.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/spring-leek-soup-gruyere-toast-recipe/