01 - Place onions, garlic, celery, carrot, bay leaf, peppercorns, thyme, and salt into a large pot.
02 - Pour in the water and heat over medium-high until boiling gently.
03 - Reduce heat to low, cover the pot, and allow to simmer for 30 minutes until onions are very tender and broth is fragrant.
04 - Remove from heat and strain the broth through a fine mesh strainer into a clean container, discarding solids.
05 - Taste the broth and add more salt if needed. Stir in lemon juice if desired.
06 - Ladle the hot broth into bowls and garnish with freshly chopped parsley.