Remove any gray membrane from the fish. Mix flour and mustard together and season with salt and pepper. Dip the fish slices in seasoned flour on one side only and arrange on a plate, floured side up.
Melt the butter in a small skillet. Add the shallots and cook on medium until both the butter and shallots have browned and acquired a nutty aroma. Do not allow them to blacken. Remove from heat, stir in capers, season with salt and pepper and set aside.
Warm 4 dinner plates or a platter. Heat olive oil in a large skillet, preferably cast iron. When very hot, add the fish medallions, floured side down, and sauté until they turn golden, no more than a minute or so. Transfer them, cooked side up, to the plates or platter.
Briefly warm the caper sauce. Add the lemon juice and tarragon and spoon a little of the sauce over each medallion. Garnish with lemon wedges and serve.