1. Heat 2 tablespoons butter in a skillet over medium high heat, reserving the remaining 1 tablespoon butter for later.

2. Generously salt and pepper both sides of the chicken breasts. When the butter is foaming, add the chicken breasts and cook about 3 - 4 minutes on each side, or, until the internal temperature of the chicken reaches 165 degrees F. If necessary, turn down the heat if the butter is browning too quickly. When the chicken is cooked, remove to a plate and cover loosely with foil.

3. Add the chopped onion to the hot pan and sauté for around 3 minutes, stirring occasionally, or until the onion begins to soften. Scrape up any brown bits from the bottom of the pan while the onion cooks. 

4. Sprinkle the flour over top of the onion. Stir the onions and flour for about 30 seconds, or until the flour has combined with the butter and onions and there are no visible white bits of flour remaining.

5. Pour in the chicken broth and stir to combine. Bring the liquid to a simmer and cook for about three minutes, stirring frequently. The sauce should thicken in this time; if it has not, cook for an additional minute or until the sauce is thickened.

6. Turn off the heat. Cut or break the remaining 1 tablespoon butter into small chunks and stir into the sauce gradually. Stir in the lemon zest and half the lemon juice. Taste the sauce and add more lemon juice if desired.

7. If using, stir in the 2 tablespoons heavy cream.

8. Taste the sauce and add more salt and pepper if needed. 

9. Add the chicken breasts and any juice that accumulated back into the pan. Rotate the chicken breasts into the sauce to coat. Garnish with thinly-sliced lemon and fresh herbs (optional). Serve immediately.