Prepare the mushrooms: Carefully remove the stems from the mushroom caps.
Cut the stems in half lengthwise, then cut into 1/4-inch / 5 mm thick slices.
Cut each mushroom cap into three equal pieces, then cut each piece into 1/4-inch / 5 mm thick slices.
Sauté the mushrooms: Heat the oil or spread in a large skillet over medium heat.
Add all the mushroom pieces and the dried marjoram.
Cover and cook for 4 minutes.
Add aromatics: Add the minced garlic and cook for 1 minute, stirring occasionally until fragrant.
Finish the dish: Remove the skillet from the heat and immediately stir in the balsamic vinegar and tamari, tossing to coat the mushrooms completely.
Don't overcrowd the skillet.
Cook the mushrooms in batches if necessary to ensure they sauté and brown properly rather than steam.
Add the balsamic vinegar and tamari off the heat. This prevents the sugars from burning and allows the residual heat of the pan to create a perfect, glossy glaze.