In a stock pot over medium heat, add 1 tablespoon butter. Add cubed chicken to pot. Brown on both sides, about 3 to 4 minutes. Remove from pan and set aside.
Add 2 tablespoons butter, onions, carrots, and celery to pan. Cook for 4 to 5 minutes, then add mushrooms and continue to cook for 2-3 more minutes, or until onions turn translucent and vegetables begin to soften and brown. Add garlic and cook for 1 more minute.
Add flour to vegetables, stir to coat. Cook for 1 minute.
Add cooked chicken, chicken broth, and half & half. Stir to combine. Cook, stirring occasionally, until warmed through and thickened, about 3-4 minutes. Remove from heat.
Optionally, top with parsley just before serving.
Chicken breasts can be substituted with cooked and shredded rotisserie chicken or canned chicken - simply skip the first step of the recipe and continue as directed.
Chicken broth can be substituted with vegetable broth.
Half & half can be substituted with whole milk (soup will be thinner) or heavy cream (soup will be thicker).
If freezing, do not add chicken (it will dry out upon reheating) or half & half (cream based soups do not freeze well). Instead, add cooked chicken and half & half while reheating soup.
Soup base can be frozen without chicken and half & half for up to 6 months in a freezer safe container or plastic bag, or refrigerated for 2-3 days.