Heat oil in a large pot over medium heat.

Add onions and one teaspoon salt. Sauté for about five minutes, until softened. Add mushrooms, garlic, zucchini, two teaspoons salt, basil, and parsley. Sauté for another 10 minutes, until softened and fragrant.

Place bones in a net bag and add to pot. Add remaining tablespoon of salt and enough water to generously cover everything. Cover pot and bring to a boil, then uncover and simmer for four to five hours (the longer the better).

Remove bag with bones from soup and set aside until cool enough to handle. Remove the meat from the bones and put back into soup. (Discard the rest.) Use an immersion blender to purée soup well, for about two to three minutes, until very creamy.