Heat oil in a large pot or Dutch oven over medium heat.
Add carrots, celery, and onion, and cook, stirring often until softened, about 8 minutes.
Stir in garlic and cook until fragrant, about 1 minute.
Add salt, pepper, Italian seasoning, and red pepper flakes.
Pour in broth and bring to a boil.
Add chicken and tortellini, and cook, stirring occasionally until chicken is warmed through and tortellini are tender, 3 to 5 minutes.
Remove from heat and stir in lemon juice.
Portion into bowls and garnish with Parmesan cheese and parsley.
