1 whole fennel bulb (about 1 pound before trimming)
1 medium onion, sliced
2 tablespoons olive oil
Salt and freshly ground pepper
2 cups cherry or grape tomatoes (or chopped tomatoes – see Kitchen Notes)
1 large clove garlic, minced
1 generous teaspoon fresh thyme leaves (or 1/2 teaspoon dried thyme)
2 cups unsalted or reduced-sodium chicken broth
2 cups water
4 ounces dry uncooked spaghetti
1. Using a sharp knife, slice off the root end of the bulb and the stalks with the fronds. Reserve the stalks and fronds. Slice the bulb in half lengthwise and peel off the tough outer layer. Cut out the inner core and slice the bulb halves in half lengthwise, then slice crosswise.
2. Heat olive oil over medium flame in a large, heavy stockpot or Dutch oven. Add fennel and onion, season lightly with salt, generously with pepper and sauté for about 5 minutes, stirring frequently. If using whole cherry or grape tomatoes, add to pot and sauté, stirring frequently, until they begin to split open, about 3 minutes (if using chopped large tomatoes, don’t add them yet). Add garlic and thyme and cook until fragrant, about 45 seconds.
3. Add broth and water. Increase heat to high and bring to a boil. If using large chopped tomatoes, add now, along with their juices. Reduce heat to simmer. Cook for about 5 minutes. Break spaghetti noodles into fourths and add to pot. Cook until pasta is cooked through, stirring occasionally. Taste and adjust seasonings. Ladle into individual soup bowls and using scissors, snip some of the reserved fennel fronds over each bowl. Serve.