Take the beef roast from the refrigerator 1 hour before cooking so it can lose its chill. A roast that is closer to room temperature cooks more evenly from the outer edge toward the center.

Pat the roast dry, rub it with the oil, then season it with 3 teaspoons of salt, 1 teaspoon dried thyme, 1 teaspoon onion powder, and 1/2 teaspoon fresh cracked pepper. The surface should look evenly coated, with no wet patches that would prevent browning.

Leave the seasoned roast at room temperature for 1 hour while you heat the oven to 325 °F. The thyme and onion powder will cling to the oil and form a fragrant crust as the roast cooks.

Place the beef fat side up in a cast-iron pan and roast for about 1 hour, until the thickest center reaches 135 °F for medium-rare. The exterior should be browned and the roast should feel gently springy when pressed.

Lift the roast from the skillet with tongs and rest it on a cutting board for at least 20 minutes before slicing. Resting allows the juices to settle back into the meat instead of spilling out during carving.

Keep the skillet as it is and warm it over medium heat after the roast comes out. The browned bits in the pan hold concentrated beef flavor for the onions.

Add the sliced onions and a large pinch of salt, then cook for 15 to 20 minutes, stirring from time to time. Halfway through, add a splash of water and scrape the dark fond from the pan with a wooden spoon as it sizzles.

Remove the onions from the heat once they are soft, deeply browned, and glossy with the skillet drippings. They should smell sweet and savory, with no raw onion bite left behind.

Whisk the mayonnaise and prepared horseradish together in a small bowl while the beef rests. The mixture should become smooth and creamy, with a sharp, clean horseradish aroma.

Spread horseradish mayonnaise over the roll bottoms, add the warm onions, then drape sliced roast beef over them and finish with dill pickle slices. Keep the pickles above the beef so their moisture does not soak directly into the bread.

Add the roll tops, cut the sandwiches in half, and serve them right away. The bread should stay structured while the beef remains juicy and the pickles stay crisp.