In a saucepan over medium heat, melt butter. Cook shallots for 3 minutes, stirring often, until soft.
Add vinegar and white wine, stirring for 3 to 4 minutes until liquid is reduced by half. Add beef stock and continue cooking until liquid is reduced by half again. Whisk in cream and simmer to thicken, about 6 minutes. Stir in mustard, parsley, and tarragon. Set aside.
Rinse meat and pat dry. Cut into half-inch-thick slices. Pour flour, salt, and pepper onto a large, shallow plate and mix. Coat both sides of liver slices with flour mixture, shaking off the excess.
Heat oil in a clean, large skillet over high heat. When oil is hot, lay a few liver slices at a time in the bottom of the skillet. Fry 3 minutes per side, or to desired doneness.
During the last batch of frying, place saucepan with sauce back on the stove and heat carefully to rewarm. Place liver slices on serving plates and spoon sauce evenly over the top. Serve with favorite side dishes.
Per serving: 562 calories, 28 grams fat (12 grams saturated fat), 24 grams total carbohydrates, 1 gram fiber, 48 grams protein.
