Step 1

Coarsely grind peppercorns and fennel seeds in a spice mill or with a mortar and pestle. Using generous three-finger pinches, season roast on all sides with kosher salt—seriously, don’t hold back, you should see salt on nearly every surface. Rub peppercorn mixture all over roast, then place on a wire rack set inside a rimmed baking sheet. Chill, uncovered, at least 12 hours and up to 2 days.

Step 2

Let roast sit at room temperature 30 minutes before baking.

Step 3

Preheat oven to 250°. Bake roast (still on rack set inside baking sheet) until an instant-read thermometer inserted into the thickest part registers 110°, 1½–2 hours.

Step 4

Meanwhile, heat 1 Tbsp. oil in a medium saucepan over medium. Working in 2 batches, cook stew meat, turning occasionally, until well browned on all sides, 6–8 minutes per batch. Transfer to a plate.

Step 5

Place onion in same saucepan, season with kosher salt, and cook, stirring occasionally, until onion is translucent and starting to brown, 6–8 minutes. Add mushrooms, increase heat to medium-high and continue to cook, stirring occasionally, until mushrooms are softened and most liquid has cooked off, about 5 minutes. Add garlic, tomato paste, Worcestershire sauce, and soy sauce and cook, stirring often, until mixture browns and begins to stick to bottom of pan, 2–3 minutes.

Step 6

Return stew meat to pan and add 6 cups water. Increase heat to high and bring to a boil. Reduce heat to medium-low (you want a good bubble going) and simmer, uncovered, until jus is reduced by two thirds, about 1½ hours.

Step 7

Using a fine-mesh sieve, strain jus into a small bowl, pressing on solids to expel as much liquid as possible, then return jus to pot (you should have 1½–2 cups). You can save the meaty, oniony solids if you’d like; it makes a great hash. Add vinegar to jus; taste and season with more kosher salt and vinegar if needed. (Be generous: The jus should taste rich and highly seasoned.) Cover jus and let sit until ready to serve roast.

Step 8

Remove roast from oven. Heat remaining 1 Tbsp. oil in a large heavy skillet, preferably cast-iron, over high until just beginning to smoke. Cook roast, turning occasionally using tongs, until deeply-browned all over, about 5 minutes total. Transfer roast to a cutting board and let rest 10 minutes (the final internal temperature should be around 120° for medium-rare).

Step 9

Meanwhile, reheat jus over medium-low. Thinly slice roast straight across against the grain and transfer slices to a platter. Sprinkle with sea salt and ground pepper. Pour about a third of jus over roast. Serve remaining jus on the side.Do ahead: Jus can be made 2 days ahead. Reheat over medium-low before serving.