Prep - Pat steaks dry with a paper towel, season both sides with salt. Crush peppercorns roughly with a mallet, back of a heavy pan, or mortar and pestle. Press one side of the steaks into the crushed peppercorns.
Cook Steak - In a large cast iron skillet, heat oil to sizzling. Add steaks, peppercorn side down and cook for 3 minutes. With a spatula, flip steaks being very careful not to break the peppercorn crust. Add butter, thyme, and garlic and cook for another 3-4 minutes, basting steaks with a spoon until well seared on the other side. Put aside on a warm dish.
Make Sauce - Pour off all but 1 tbsp of fat from the pan. Add shallots and unused crushed peppers. Turn heat on to medium-high to cook for 2 minutes. Add Chambord and cook until alcohol has burned away and it's almost totally reduced, 1-2 minutes. Pour in chicken broth and bring to a simmer, scraping up browned bits along the way. Whisk in cream and reduce sauce a bit. Whisk in mustard and season with salt.
Server - Return steaks to pan and spoon sauce over them. Arrange steaks on a platter or individual plates and top with remaining sauce. Serve with pomme frites or garlic-mashed potatoes and steamed green beans.