Season the chicken. Pat the chicken dry and season with 1 teaspoon salt and 1/2 teaspoon black pepper. This helps it brown and stay flavorful.
Sear the chicken. Heat 1 tablespoon olive oil and 1 tablespoon butter in a large pot over medium-high heat.
Add the chicken in a single layer and cook until lightly browned, 4–6 minutes. Remove to a plate.
Brown the mushrooms. Add another tablespoon of oil and 1 tablespoon butter to the pot. Stir in the sliced mushrooms, a pinch of salt, and cook undisturbed for 2 minutes to get color.
Continue cooking, stirring occasionally, until deeply browned and their liquid has evaporated, 6–8 minutes. Transfer half to a plate for garnish if you like.
Sauté the aromatics. Reduce heat to medium. Add the onion, carrots, and celery with a pinch of salt.
Cook until softened, 5–7 minutes. Stir in the garlic and thyme and cook 30 seconds until fragrant.
Make the roux. Sprinkle the flour over the vegetables and stir well to coat. Cook 1–2 minutes to remove the raw flour taste.
Add remaining 1 tablespoon butter if the pot looks dry.
Deglaze. Pour in the sherry or wine, scraping up any browned bits. Let it bubble for 1 minute to cook off the alcohol.
Add stock and simmer. Gradually whisk in the chicken stock to prevent lumps. Add the bay leaf and return the seared chicken (and any juices) to the pot.
Bring to a gentle simmer and cook 15–20 minutes, until the chicken is tender and flavors meld.
Finish with cream. Lower the heat. Stir in the heavy cream and the reserved browned mushrooms (if using). Simmer gently for 3–5 minutes.
Do not let it boil hard, or the dairy may separate.
Adjust seasoning. Stir in the lemon juice. Taste and add more salt and pepper as needed. If you prefer a thicker soup, simmer a few minutes longer; for thinner, add a splash of stock.
Optional starch. If adding cooked rice, noodles, or parboiled potatoes, stir them in now and warm through for 3–4 minutes.
Serve. Remove the bay leaf.
Ladle into bowls and garnish with chopped parsley and a crack of black pepper.