Prepare the pickled mushrooms: warm one tablespoon olive oil in a medium-size pan over medium heat. Add the onion and cook for a few minutes. Add the garlic and mushrooms and cook for about 6-7 minutes, until soft. Remove from heat and set aside to cool. In a jar or bowl (that seals with a lid), combine the vinegar, salt, oregano, and mustard powder. Seal and shake to combine, or whisk well. Add the cooled mushrooms, seal with a lid and refrigerate for at least 4 hours or up to overnight.Assemble the bowls: divide the cooked rice among four separate bowls then top with pickled beets, mushrooms, diced cucumber, sliced tofu, avocado and pickled ginger. In a small bowl, whisk together lemon juice and tamari. Pour over top of the bowls, serve and enjoy!