Bring four quarts of water to the boil with one tablespoon of salt, or to taste, and add the penne. Cook, stirring down with the fork, for 10 to 12 minutes.
03Meanwhile, place a cast-iron skillet over high heat and sprinkle the pan with one tablespoon of kosher salt. When the salt begins to sputter, add the tuna steak, sprinkle the top of the steak with more salt and cook until lightly charred, about three minutes per side. Transfer the steak to a cutting board.04Drain the pasta and toss with the remaining tablespoon of olive oil. Divide the pasta and sauce among four dishes. Slice the tuna into quarter-inch slices and place over the sauce. Serve immediately.