French Dip Roast Beef Sandwiches with Thyme Jus

Heat the oven. For the Roast Beef plus the Jus: Set roast on a rack set in a rimmed baking tray or on a platter and scatter uniformly with the 4 teaspoons salt and chill uncovered for at least 1 hour or upwards to 24 hours. Adjust oven rack in order to middle position plus heat oven to 225℉ (105℃). Mix dried mushrooms with very hot tap water and (1 cup) leave to sit till mushrooms are softened, around 5 minutes. Tip off the liquid mushrooms and squeeze dry, next mince and keep to one side. Pat roast dry using paper towels, if needed, and scatter with the 1 black pepper (1/2 teaspoons). In a 30 cm stainless steel or cast-iron skillet, heat oil over a medium-high flame till only just smoking. Brown roast upon all sides, 8 to 12 minutes; return to rack or platter. Pour off all but around 2 tablespoons fat from skillet (or, if you need to, add extra oil if you need to to equal 2 tablespoons). Tip in onions and butter and cook till onions are starting to soften and are gently browned, 5 to 7 minutes. Mix in this minced porcini mushrooms, flour, tomato paste, plus thyme plus heat till mixture is well mixed and begins to darken, around 2 minutes. Take out from heat and push the onion mixture in order to the center of the skillet. Stand the roast, fat side up, upon top of the onion mixture. Move the skillet to the oven and roast till an instant-read thermometer inserted in the middle of the roast registers 125℉ (50℃) for mid-sized-rare, 1 to 1 1/2 hours. Take it out of the oven and move the roast to a carving board; leave till it is cold to room temperature, around 1 hour. Using a potholder, set the skillet using the onion mixture over a mid-sized flame and cook, mixing now and then, till deep golden brown, 3 to 5 minutes while that is happening. Slowly beat in the broth plus Worcestershire sauce, scraping up any browned bits off the bottom of the pan, bring to a gentle simmer, and cook, mixing now and then, till the jus is a little thickened, around 15 minutes. Mix in any accumulated meat juices from the rested roast. Stand the jus aside off heat till ready to assemble the sandwiches. (the roast beef can be covered and the jus moved to an airtight container and both chilled separately for up to 4 days before slicing plus assembling this sandwiches.) For Assembling the Sandwiches: Adjust the oven rack in order to upper-middle position and heat the oven broiler. Take out the kitchen twine from the roast plus slice the roast as thin as possible against this grain; keep to one side. Reheat the jus over a medium-high flame till only just simmering, next reduce heat to low to keep warm. Dip this cut side of this roll bottoms (just the bottoms for now) in the jus to saturate using tongs and your hands. Move the dipped roll bottoms, cut side up, in order to a baking tray. Mix the sliced beef using any accumulated juices in the warmed jus in the skillet plus leave to sit till heated through, around 1 minute. Share the dipped roast beef uniformly among the dipped roll bottoms using tongs. Layer 2 slices Swiss cheese over this beef on every roll. Move the baking tray with the open sandwiches to the oven and broil till the cheese is just melted, around 1 minute. Set baking tray with the half-assembled sandwiches aside. Dip the cut side of the roll tops in the jus to saturate, next close each sandwich using tongs. Pour the leftover jus into individual serving cups, around per serving (1/2 cup). Serve sandwiches with the jus.