Peel the garlic close and crush the cloves through a garlic press. Place in a small bowl.
Preheat a 6-qt. (or larger) stock pot or Dutch oven over medium heat until the rim of the pan is hot to the touch. Add the oil to the pan and heat for 2 - 3 minutes more.
Add the onions and sauté until translucent.
Add the garlic and cook for 3 - 5 minutes more, being careful not to brown or burn the garlic.
Add the dried thyme. Season with salt and pepper. Add all the stock and bring to a boil. Reduce the temperature to low and simmer the soup for at least 1 hour.
Add the bread cubes to the soup and cook until the bread is very soft and tender.
Let the soup cool for 15 to 20 minutes.
Use an immersion blender or carefully transfer it in batches to a blender or food processor to blend until smooth. If the soup seems too thin, adjust the consistency by adding additional bread cubes and blending again.
Add the heavy cream and stir to incorporate. Add salt and freshly ground black pepper to taste.
Return the pan to the burner and cook over very low heat until heated through.
Garnish each bowl with a sprig of fresh thyme and a few toasted croutons. Serve immediately.