2 tablespoons extra-virgin olive oil
2 medium leeks, white and tender green parts only, thinly sliced
1/2 pound cremini mushrooms, thinly sliced
Salt and freshly ground pepper
1 pound skinless, boneless chicken breast halves, cut into 2-inch pieces
All-purpose flour, for dusting
1 1/2 cups chicken stock or low-sodium broth
1 tablespoon chopped thyme
2 tablespoons sour cream
2 teaspoons Dijon mustard