Dredge liver slices in flour, shaking off excess, and season with salt and pepper.
Melt butter in a large skillet over medium high heat. Cook liver for 5 minutes per side, reducing heat if necessary. Transfer to a heated serving dish, and keep warm.
In the same skillet, increase heat to medium high and add tarragon vinegar, scraping the bottom of the skillet with a wooden spoon. If necessary, add a small amount of water to increase sauce. Lay lemon slices over liver, and pour sauce over the top. Sprinkle with freshly chopped parsley. Serve immediately.