Cover the chicken breast with plastic wrap and gently pound it to an even thickness. Season all over with salt and pepper.
Warm the olive oil in a large pot or dutch oven over medium-high heat. Once hot, add the chicken breast and sear on both sides until golden, about 2-3 minutes per side. Remove the chicken to a plate, but do not wipe the pan clean. (The chicken does not need to be cooked through)
Add the onions to the pot along with the butter. Cook, stirring occasionally, for about 5 minutes, then add the garlic and cook for another minute.
Reduce the heat to medium and sprinkle the flour over the onions. Cook, stirring frequently, for about 2 minutes. Add the chicken stock and to the pot and mix, then add the half-and-half.
Bring the soup to a gentle simmer, and reduce heat if needed. Add all of the seasonings. Dissolve the cornstarch in the warm water and add it to the pot. Add the chicken back to the pot and let it simmer for 20-25 minutes until the soup has thickened, and the chicken is cooked through.
Remove the chicken and the bay leaves from the pot. Add the carrots and the tortellini to the pot. Shred the chicken while the tortellini cooks.
Once the tortellini have cooked through (about 3 minutes) remove the pot from heat and add the shredded chicken and spinach. Mix together until the spinach is slightly wilted, then serve immediately.