Remove the cooked chicken or pork from the skillet. Working over medium-high heat, melt the butter, add the thyme and shallot, and cook for about 2 minutes. Sprinkle the flour into the pan, then cook for 1 minute. Whisk in the chicken stock, bring it up to a bubble, and cook until thickened, 2 minutes. Add 2 teaspoons of lemon zest and the juice of the zested lemon, and season with salt and pepper. Pour the sauce over the chicken or pork and serve.