Sometimes, figuring out what to cook for dinner takes longer than actually cooking it. I can't tell you how many times I've gotten excited about a recipe, then realized that it requires marinating for twelve hours. When it's already 6 p.m., that just isn't going to work.
A lot of times, a lengthy marinating process has as much to do with texture as it does flavor. Marinating tenderizes a tougher cut of meat because it helps break down the connective tissues that make it tough. If you want to make a specific recipe, but don't have time to marinate it properly, a simple, unexpected solution is fruit.
Some fruits, like kiwi, pineapple, and papaya, contain enzymes that break down the collagen in meat, making it much more tender. Redditor DaytonFlyers suggests peeling and mashing a kiwi fruit, then spreading the pulp over the surface of the meat. After about fifteen minutes, it will be significantly more tender.
And you don't need much—at all. According to The Kitchn, one half of a kiwi fruit is enough to tenderize four to five pounds of meat. You also want to be careful not to leave it on for too long, unless you want it to turn to mush.
But this is why kiwifruit is the best choice over the other fruits; its enzyme, Actinidin, is not as strong as the enzymes in pineapple or papaya (Bromelain and Papain, respectively), so it will tenderize the meat without making it too mushy if you leave it on longer. Plus, kiwifruit has a somewhat neutral flavor, so it won't overpower the taste of the steak. (Bonus: kiwis are also great for digestion, so save one for after your meal!)
If you're new to kiwis, the easiest way to peel one is with a spoon. Just cut off the ends, then use a spoon to separate the skin from the fruit. Once you've peeled it, you can puree the fruit in a food processor, or just mash it with your hands or a fork.
For those who are allergic to kiwi (which is actually a lot more common than you'd think), you can use fresh pineapple or papaya juice or puree for similar results—just make sure you don't leave it on too long. If you're already planning on marinating the meat for a short time, you can just add the juice or puree to the marinade instead.
If you don't like the idea of using fruit to tenderize your steak, you can use coarse salt instead, though the marinating time will be slightly longer.