Half or quarter any very large mushrooms. Bring a large pot of water to a boil.
Meanwhile, chop the shallot, garlic, parsley and fresh thyme. Place in a medium bowl and mix with the olive oil, red wine vinegar, Dijon mustard, onion powder, red pepper flakes, sugar and salt.
Once the water is boiling, add the mushrooms, and boil for 3 minutes. Drain the mushrooms in a colander and rinse them under cool water until they are warm to the touch, rinsing out any dirt that has clung to the mushrooms.
Place the warm mushrooms in the marinade. Transfer to a sealable container and refrigerate for at least 30 minutes, but preferably 2 to 3 hours if time (the flavor is very vinegary at first but rounds out as it chills). Stores up to 1 week; bring to room temperature before serving (the olive oil solidifies when refrigerated).