Heat 2 tablespoons of the butter in a sauté pan over medium heat (preferably the same pan you used to cook the chicken or fish you are saucing), and sauté the shallots or onions until softened but not brown, about 2 minutes. Add the stock and lemon juice, and bring to a boil over high heat, stirring regularly and making sure to scrape up all the brown bits. Continue to cook until the liquid is reduced by half and lightly thickened, 3 to 5 minutes. Remove the pan from the heat, whisk in the remaining 2 tablespoons of butter a bit at a time to thicken the sauce, and then the herbs. Taste and season with salt and pepper if you think it needs it. Serve immediately, spooned over chicken or fish.