Heat the oil in a large pot over medium-high heat. Add the onion and sauté to soften slightly, 4 to 5 minutes.
Add the Vegetables, Protein (except shrimp and scallops, which are added during the last 5 minutes of cooking), Starch, tomatoes, broth, and Flavorings (if using dried herbs, woody fresh herbs, or spices).
Bring to a simmer, reduce the heat to medium-low, and simmer, partially covered, until the vegetables are tender and the flavors have blended, 15 to 20 minutes.
Add any Flavorings and extra broth or water if needed: The soup should be thick, but juicy. Adjust seasonings, including salt and pepper to taste. Serve.
TIPS
If you prefer the green soup vegetables brightly colored rather than fully cooked, add them during the last 5 minutes of cooking rather than with the rest of the vegetables. If you prefer the green soup vegetables fully cooked but want bright color, add a handful of chopped fresh herbs, such as parsley, basil, or cilantro, at the end of cooking. To make the soup vegetarian, substitute vegetable broth for chicken broth, omit the meat, and add a second starch (grains, beans, or potatoes). To decrease the amount of meat and use it just as flavoring, increase the vegetables from 1 to 1½ pounds and decrease the meat from 1 pound to ½ pound. Vegetable weights don’t need to be exact, but allow a little more than a pound, so by the time you’ve trimmed them, you’ve got close to a pound. Time permitting, boost a shrimp-based soup’s flavor by simmering the shrimp shells in some of the broth for a few minutes. Strain them out before adding the broth to the soup. For quicker chicken soup, use 2 to 3 cups shredded rotisserie chicken instead of chicken breasts or thighs.