Chill the steak in the freezer for about 30 minutes, then use a sharp knife to cut it into very thin slices. Slice across the visible grain lines so the finished beef is easier to bite through.

Stir the soy sauce or coconut aminos, brown sweetener, sesame oil, garlic, and ginger paste together in a large bowl. The mixture should smell savory and gingery, with the sweetener beginning to dissolve.

Add the sliced steak and turn it through the marinade until every strip is coated. The beef should look glossy rather than sitting in isolated pockets of seasoning.

Cover the bowl with plastic wrap and marinate for 2 hours, turning the beef occasionally with tongs. Regular tossing redistributes the aromatics so the slices season evenly.

Warm 1 tablespoon of avocado oil in a skillet over medium-high heat. The oil should look fluid and shimmer before the first batch goes in.

Sear the beef in 3 to 4 uncrowded batches, cooking each batch for 2 to 3 minutes per side and adding more oil as needed. Listen for a strong sizzle and look for dark caramelized edges instead of pale meat steaming in liquid.

Transfer the seared beef to a serving dish and finish with the toasted sesame seeds. The sesame fragrance should bloom over the warm, browned beef.