Much of this recipe is malleable, but the ratio of salt to water is not: Too much and you kill any ferment, too little and everything can rot. Your nose is a good guide. If your fermented greens stink like rot, don't eat them. They should have a pleasing pungent smell like a cross between mustard and dill pickles. Once the greens have fermented to your liking, seal the jars and store in the fridge. They'll last this way for a year or more.
Bring everything but the salt and mustard greens to a boil in a large pot. Turn off the heat and let it cool to room temperature. Pour everything into a vessel that you can weigh, and weigh the water plus spices in grams. Write this down.
Wash you mustard greens well, then shake them dry and weigh them in grams. Write this down.
When it's cool, weigh out 2 percent of the total weight of greens + water + spices in salt. Dissolve this in the liquid. If you use fine-grained salt you should have no problem. If the salt doesn't want to dissolve, stir until it does.
NOTE: If you don't want to weigh all this out, just use the salt ratio I have in the ingredients list. It is approximately the same, but not exactly.
Get out your fermenting crock or 5 quart-sized Mason jars with 5 narrow jelly jars to keep the greens submerged. You need this many because you will only pack the greens in 3/4 of the way into the jars -- you want at least 1 inch of brine above the level of the greens.
Submerge the greens in the brine, using a chopstick or skewer to get rid of any air bubbles. Put the weights on the greens if you have them. If not, place the Mason jars in the sink and put the narrow jelly jars in them. The brine will overflow but the narrow jar will prevent the greens from contacting the air. Set the jars on a baking sheet and put in a cool place away from direct sunlight.
Let them ferment at least 3 days, or longer. Mold may form eventually. This is normal. I wait until the mold cap is pretty solid, then pick it off. It's not harmful unless it's black.
Finish by packing jars tight with the fermented greens, leaving about 1/2 inch of brine over them. Tighten the lids and put in the fridge. They will continue to ferment very slowly, so open the jars every week or two to release pressure. If you want to kill the ferment, boil the brine and cool before packing the jars the final time.
Note that prep time does not include fermenting time.