Look into lacto fermentation (home made pickles like real sauerkraut).

Basically make sure your jar is as clean as possible; chop up a little bit of cabbage, carrots, cauliflower, broccoli (any or all, this is just an easy beginner list, the possibilities are endless); jam it down in the jar leaving a good gap at the top, and packing it in to leave as little air as possible; top up with clean, salty water so the veggies are covered (not crazy loads of salt, but the water should taste like between blood and sea water) (can also use a weight to keep the veg submerged if necessary, like a clean glass with water in); cover, leave at room temp a few weeks; enjoy a delicious and healthy food that humans have maybe been doing since prehistory. I was going to say how it works, but ran out of time, look it up!