Look for a kombucha that isn’t sweetened with artificial sugars (I like Health-Ade). Before you open the bottle, slowly turn it upside down, then back up, then slowly open the bottle—I’ve found that most of the gingery bits tend to settle at the bottom, and you want all that flavor in your marinade. But whatever you do, don’t shake the bottle to mix up the drink, unless you’re in the mood for a gingery shower. —Rebecca Firkser
Place the steak in a large resealable plastic bag, then season with a few good pinches of kosher salt.
In a medium bowl, whisk together the kombucha, soy sauce, rice vinegar, grated garlic, oil, and red pepper flakes. Pour all but 1/2 cup of the marinade into the bag with the steak. Seal the bag, pressing out air along the way, and massage the bag to disperse the marinade. Set the bag aside and let marinate for 20 minutes at room temperature or up to 2 hours in the fridge.
Add the sliced shallots to the remaining marinade and let sit for at least 30 minutes.
Meanwhile, place the cucumbers on a cutting board and gently apply pressure to crush them with a rolling pin or the side of a large knife (don’t squash them completely). Break or slice the cucumbers into bite sized pieces and scoop them into a sieve or colander placed over a medium bowl. Toss cucumbers with a few good pinches of salt and let sit for at least 30 minutes or up to 2 hours. Discard any liquid from the bowl, wipe it out, then dump the cucumbers into the bowl. Toss the cucumbers with the shallot-y marinade mixture and gochujang. Set aside.
Heat vegetable oil in a large cast iron or stainless steel skillet over medium high heat. Remove steak pieces from bag and pat dry with paper towels. Discard leftover marinade. Cook steaks (in two batches to avoid crowding the pan), turning occasionally, until they’re deep brown on both sides and register 130ºF on an instant-read thermometer (about 8 minutes for medium rare). Transfer the steaks to a cutting board and let rest for at least 10 minutes.
Slice steak thinly against the grain and transfer to a serving platter. Top with flaky sea salt, and serve with smashed cucumber salad and rice if you’d like.
Rebecca Firkser is a freelance food writer and recipe developer. Her work has appeared in a number of publications, among them Food52, TASTE, Edible Manhattan, Extra Crispy, The Strategist, and Bon Appetit's Healthyish. She contributed recipes and words to the book "Breakfast: The Most Important Book About the Best Meal of the Day." Once upon a time, she studied theatre design and art history at Smith College, so if you need a last-minute avocado costume or want to talk about Wayne Thiebaud's cakes, she's your girl.