2 large onions, coarsely chopped
2 stalks celery, coarsely chopped
2 carrots, peeled and chopped
1 bunch scallions, chopped fine
15 cloves garlic, peeled and smashed
1 tsp. olive oil
1 tsp. salt
5-6 quarts water
1 head cauliflower, cored and cut into chunks
1 tomato, chopped
8 sprigs fresh thyme
3 bay leaves
1 tsp. black peppercorns
Add the onions, celery, carrots, scallions, garlic, oil, and salt to a large Dutch oven or stock pot and stir together. Cover and cook over medium-low heat, stirring often, until the pan bottom begins to brown lightly, 20-30 minutes.
Add in the water, cauliflower, tomato, thyme, bay leaves, and peppercorns. Partially cover, bring to a gentle simmer (not a boil) and cook for 1½ hour, adjusting the heat as needed to maintain a gentle simmer.
Strain the broth through a fine mesh strainer into a pitcher and pour into storage containers. Use within 3 days or freeze until ready to use.