For the steaks: Heat 2 cast iron skillets large enough to hold 2 of the 4 pieces of beef until they begin to visibly smoke. Season the steaks on both sides with salt and pepper. Shut the heat off underneath the skillet and use a pair of tongs to place the steaks in the pan 1 by 1 in a single layer. Turn the heat on under the skillet and brown the first side, 3 to 5 minutes. Try to resist the temptation to turn them over or move them as they cook.
Turn the steaks on the second side and brown, 3 to 5 minutes.
Arrange the steaks on a rack to "rest" while you make your sauce.
Remove the excess grease from 1 of the pans that was used to cook the steaks and reserve the other pan for the potatoes. Add 1 tablespoon of butter and the shallots and cook until they are translucent, 3 to 5 minutes. Add the green peppercorns and brandy and cook until the liquid reduces almost completely.
Stir in the mustard, lemon juice, lemon zest and Worcestershire sauce. Swirl the pan so all of the flavors start to meld together.
Add the veal stock and reduce to a desirable consistency, then stir in the tarragon.
For the potatoes:
Preheat the oven to 375 degrees F.
Add the potatoes to a casserole with a fitted lid that is just big enough to hold the potatoes. Toss them with some sea salt and the bay leaves. Cover the casserole and put in the center of the oven to bake for 25 to 30 minutes.
When the potatoes are tender when pierced with the tip of a knife, remove the casserole from the oven and discard the bay leaves.
Heat the reserved skillet that the steaks were cooked in over medium heat and add the potatoes into the steak drippings. Cook until they brown and become slightly crisp. slightly. Season with salt, to taste.
Arrange the potatoes on the bottom of 4 dinner plates and top with the steak. Drizzle with the sauce and serve immediately.