Warm the olive oil in a large soup pot over medium-high heat, then cook the onion for 5 to 7 minutes. The onion should soften and pick up a little golden color around the edges.

Stir in the celery, carrots, garlic, and Italian seasoning, and cook for a few more minutes. When the garlic smells fragrant and the vegetables look glossy, the aromatics are ready for broth.

Pour in the chicken broth and bring it to a boil over high heat, then lower the heat to a rapid simmer for 10 minutes. The carrots and celery should lose their raw crunch and yield easily when tasted.

Add the shredded chicken and refrigerated tortellini, then raise the heat to medium-high until the soup returns to a lively simmer. Cook until the tortellini are tender and begin floating softly; lower the heat right away if the bubbling becomes forceful.

Taste and season with salt and pepper, then finish with chopped parsley and serve right away. The broth should taste savory and balanced, with intact tortellini and warm chicken in every spoonful.