Amanita Muscaria mushrooms seasoned with salt and fermented in brine inspired by the Traditional Japanese technique. You can also pickle the mushrooms with vinegar for a fine result if you aren't experienced with fermenting.
Remove the mushrooms and dry them very well then put them in a glass jar.
Put the jar on a scale, add water to cover the mushrooms by ½ inch, weighing the water and mushrooms in grams as you pour it in. Multiply the grams by .03 or 3%, then add that many grams of salt to the jar. A pint jar is a good place to start, for that, you can use a tablespoon of kosher salt, or roughly 13 grams. Also remember that a pint is one pound, or 448 grams, and 448 x .03 = 13.44 grams of salt. (You can use anywhere from 2-5% salt here for reference).
Shake the jar, weigh the mushrooms down with a clean stone or clingfilm, screw on the lid, and leave in a dish to catch any liquid at room temperature for 5 days, and up to 10, or until they're soured to your liking. The mushrooms should be covered with liquid the entire time.
After that, refrigerate the mushrooms. The mushrooms will continue to age a bit in the fridge. If you want a stronger flavor, leave them out for another day or two, tasting them regularly.
They'll last for a long time if always kept completely submerged in liquid, and will continue to develop as they age.
Notes
Fermenting without a vacuum bag
You can also pack the mushrooms into a jar with a weight on them as you would sauerkraut, if you want to do that, tare your fermentation container on a scale to 0, then add the mushrooms and water to cover. Multiply the combined weight in grams of the mushrooms and water by .03, then add that amount of salt in grams, stir to dissolve, and put a weight on the mushrooms to keep them under the brine, then ferment per usual.