Zest and juice lemon, reserving zest for later. Combine the lemon juice, garlic, kosher salt and pepper. Coat porterhouse steak evenly, place in a large zipper-locking plastic bag with rosemary, pressing out all air or wrap tightly in plastic wrap; refrigerate overnight.

Preheat grill to warm (250°F).

Wrap porterhouse in foil, place in grill away from direct heat and cook slowly for 60-90 minutes, flipping once (internal temperature target should be 100F). Remove steak from grill and increase grill heat to high (450°F-500°F). Allow steak to rest 15 minutes in foil while grill is heating to higher temperature.

Remove steak from foil, pat dry, brush with olive oil and sear 2-3 minutes per side for medium rare (125°F-130°F). Transfer to a clean cutting board. Combine reserved lemon zest and sea salt.

Carve steak by running knife along the bone to remove both the strip and tenderloin. Slice each steak in 1/4 to 1/2-inch thick slices and serve on platter, garnished with lemon sea salt.