Prepare mushrooms by rinsing them in cool water. Slice the mushrooms into 1/4" pieces. Or if using baby shiitakes as in post, no need to slice them.
In a medium-sized pot, toss mushrooms and fill with water. Bring to boil. Reduce heat to medium-high and cook for 10 minutes.
Strain mushrooms in a colander and rinse with filtered water.
In a medium bowl, combine mushrooms, salt, peppercorns, bay leaves, sliced garlic, apple cider vinegar, lemon juice, olive oil and sliced onions. Mix well to incorporate all flavors. Can be consumed right away but it does become more flavorful after a few days.