Place a soup pot over medium heat and add the onions, celery, carrots, leeks, garlic, thyme and bay leaves. Sweat vegetables covered for 10 minutes, stirring regularly. Add the chicken stock and simmer for one hour.
Remove bay leaves and thyme and add frozen peas. When peas are cooked (1-2 minutes), add fat-free or low-fat lactose-free milk and bring to a boil. Remove from heat and carefully puree in a blender. Add nutmeg and ham cubes and salt or pepper to taste. Ladle soup into bowls and garnish with croutons if desired.