Step 1

Toast Sichuan peppercorns, coriander seeds, and cumin seeds in a dry small skillet over medium heat, tossing often, until spices are very fragrant and slightly darkened in color, about 3 minutes. Transfer to a spice mill and let cool; finely grind.

Step 2

Pat steak dry; season all over with salt. Whisk oil, oyster sauce, fish sauce, and spices in a large bowl. Add steak; turn to coat. Cover and chill 4–12 hours (for the most flavor go the full time!).

Step 3

Remove steak from marinade and place on a plate. Let sit at room temperature 1 hour before cooking.

Step 4

Prepare a grill for medium-high indirect heat (for a charcoal grill, bank coals on one side of grill; for a gas grill, leave one or two burners off). Grill steak over direct heat until lightly charred, about 2 minutes per side. Move steak over to indirect heat and cover grill, placing vent (if your grill has one) over steak so it draws heat up and over it. Grill, turning steak occasionally, until an instant-read thermometer inserted into the thickest part of steak registers 120°, 20–25 minutes for medium-rare.

Step 5

Transfer steak to a cutting board and let rest 15 minutes before slicing it against the grain.

Step 6

While the steak is resting, place a wire rack on direct heat side of grill, arranging so bars run perpendicular to grate. Toss tomatoes with oil in a medium bowl; season with salt. Grill tomatoes in a single layer on rack, turning occasionally, until lightly charred all over and beginning to burst, about 5 minutes. Return to bowl as they’re done and let cool.

Step 7

Pulse shallot, garlic, chiles, and a pinch of salt in a food processor until mixture starts to resemble a paste. (This is even better in a mortar and pestle, which will create a finer texture.) Add cilantro and pulse until very finely chopped.

Step 8

Lightly crush tomatoes with your hands and add shallot mixture, lime juice, fish sauce, and oyster sauce. Season sauce with salt; let sit 10 minutes. Serve steak with sauce, rice, and lettuce.