Heat a medium saucepan over medium heat. Add the olive oil, celery, carrot and shallots and cook, stirring often, about 5 minutes, or until softened.
Add the thyme, 1/4 teaspoon salt, the parmesan rind, chicken stock and 1 cup water. Bring to a simmer and cook for 10 minutes.
Add the pasta and stir with a wooden spoon to prevent it from sticking to the pan. Return to a simmer and cook 8 to 10 minutes more, or until the pasta is cooked and the liquid is slightly thickened.
Remove the thyme sprigs and parmesan rind. Stir in the peas. Ladle the soup into bowls and top with grated parmesan and lemon zest, if desired.
Photograph by Con Poulos
Courtesy of Food Network Magazine