Coat liver slices with flour, shaking off excess, and season with salt and pepper.
Melt butter in a large frypan 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 frypan increase heat to medium high and add tarragon vinegar, scraping the bottom of the frypan 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.