Spray a large skillet with nonstick spray. Place over medium-high heat and add the chicken breasts. Cook for 3 minutes, or until lightly browned; turn over and cook on the other side until lightly browned, 3 more minutes. Sprinkle the lemon juice over the chicken, cover the pan and reduce the heat to medium-low. Cook for about 10 more minutes, or until no longer pink inside.
Remove the chicken to a plate and cover with foil.
Turn the heat back up to medium-high. Add the olive oil and the onion. Pour enough of the broth or water into the pan to deglaze. Scrape all the browned bits off the bottom of the pan. Add the mushrooms and garlic, and cook for 4 to 5 minutes. Add the sherry and bring to a boil. Dissolve the cornstarch in the half-and-half and add to the mixture. Add the salt and pepper, and bring to a boil, stirring constantly, until thickened.
Serve immediately over the chicken breasts.
Per serving: 318 calories, 7 g fat, 86 mg cholesterol, 438 mg sodium, 18 g carbohydrates, 34 g protein, 2 g fiber.