Poach the chicken. If using cooked chicken, skip to step 2. For raw chicken, place the breasts in a saucepan, season with salt, and cover with water by 1 inch. Bring to a boil over medium-high heat, then reduce to a gentle simmer. Cover and cook for 10 to 12 minutes, skimming off any foam that rises to the surface. The chicken is done when the internal temperature reaches 170°F. Remove, let cool for a few minutes, then chop into bite-size pieces.
Sauté the vegetables. Heat the olive oil in a 2-quart saucepan over medium-high heat. Add the chopped onion, carrot, and celery. Cook for about 8 minutes, stirring occasionally, until softened and just starting to turn golden at the edges. Add the garlic and thyme and cook 1 minute more, until fragrant.
Build the broth. Pour in the chicken broth and bring to a boil. Reduce the heat to a simmer, add the chopped chicken, and cook for 10 minutes so the flavors come together.
Cook the pasta. Add the pasta and cook, stirring occasionally, until tender, about 5 minutes. Cook time will depend on the pasta shape you're using.
Taste and season. Add extra salt and freshly ground black pepper to taste. Serve hot.
Notes
Use a heavy-bottomed saucepan. A 2-quart saucepan with a thick bottom distributes heat evenly and keeps the vegetables from scorching while they soften.Don't rush the mirepoix. The full 8 minutes of sautéing the onion, carrot, and celery is where most of your flavor comes from. If they're still pale and crunchy, keep going.Cook the pasta based on when you'll eat it. If you're eating the soup right away, cook the pasta in the broth so it releases starch and gives the soup more body. If you're saving half for leftovers, cook the pasta separately and combine in each bowl as you serve, since pasta left in broth turns mushy overnight.Taste before you salt. Chicken broth varies wildly in sodium. Always taste the finished soup before adding salt.If doubling the recipe, use a 3 or 4-quart pot.