Bring a large pot of water to a boil. Add chicken breasts. Boil for 20-25 minutes until the center is no longer pink.
While the chicken cooks, dice the onion and slice the carrots.
Once the chicken is done cooking, set it aside and shred. Pour out the cooking water from the pot. Using the same pot over a medium heat melt the butter. Once melted, sauté the onions and carrots until transclucent (about 5 minutes). Add minced garlic, stir, and cook for about a minute.
Add the flour. Stir for about 2-3 minutes to cook out the floury taste.
Add the chicken broth, half and half, and spinach. Stir until the soup thickens slightly, about 5-7 minutes.
Add the shredded chicken, salt, pepper, and red pepper flakes (if using). Stir well. Cook for 8-10 minutes, stirring occasionally to bring the flavors together.
Prep the tortellini according to package instructions, and prep the garlic toast (see notes).
Add parmesan to the soup 1/2 cup at a time, stirring and allowing it to melt slowly.
Keep tortellini separate from the soup until ready to serve. To serve, put tortellini in bowl first, then ladel the soup over. Serve with Easy Garlic Toast.