Season chicken with pepper and salt on both sides.
Heat oil in a medium skillet over medium heat. Add the chicken and cook, turning once or twice and adjusting the heat to prevent burning, until brown and an instant-read thermometer inserted into the thickest part registers 165°F, 12 to 16 minutes. Transfer to a plate and tent with foil to keep warm.
Add shallot to the pan and cook, stirring, until fragrant, about 30 seconds. Add mushrooms; cook, stirring occasionally, until tender, about 2 minutes. Pour in vermouth (or wine); simmer until almost evaporated, scraping up any browned bits, about 1 minute. Pour in broth and cook until reduced by half, 1 to 2 minutes. Stir in cream and chives (or scallions); return to a simmer. Return the chicken to the pan, turn to coat with sauce and cook until heated through, about 1 minute.
Tip: It's difficult to find an individual chicken breast small enough for one portion. Removing the thin strip of meat from the underside of a 5-ounce breast—the “tender”—removes about 1 ounce of meat and yields a perfect 4-ounce portion. Wrap and freeze the tenders and when you have gathered enough, use them in a stir-fry or for oven-baked chicken fingers.
Per serving: 275 calories; 15 g fat ( 5 g sat , 7 g mono ); 84 mg cholesterol; 5 g carbohydrates; 25 g protein; 1 g fiber; 373 mg sodium; 370 mg potassium.