How to Make It
Preheat the oven to 300°F. Line a large baking sheet with parchment paper.
Prepare the mushrooms by removing the stems and cleaning with a damp cloth or paper towel. Then slice the mushrooms.
Toss the sliced mushrooms with the olive oil and tamari until well coated and place on the baking sheet so that the mushrooms are not touching one another. Bake for 30 to 40 minutes, tossing halfway through, until the mushrooms are shriveled up and crispy.
Assemble the sandwiches on the bread with a slather of mayo, arugula, sliced tomatoes, and crispy shiitake mushrooms.