2pounds boneless, skinless chicken breasts, thighs, or a combination
Table salt and ground black pepper
1tablespoon unsalted butter
1tablespoon olive oil
1pound cremini mushrooms, stems trimmed, caps wiped clean and cut into 1/4-inch slices
1 medium onion, chopped fine (about 1 cup)
1/4cup dry white wine
1tablespoon unbleached all-purpose flour
1 medium garlic clove, minced or pressed through garlic press (about 1 teaspoon)
1 1/2cups low-sodium chicken broth
1/3cup sour cream
1 egg yolk
1/2teaspoon freshly grated nutmeg
2teaspoons juice from 1 lemon
2teaspoons minced fresh tarragon (see note)