GLUTEN-FREE OPTION, NUT-FREE, SOY-FREE
You can have some fun with this one, switching up a few ingredients (swap lime juice for vinegar, avocado oil for olive oil, and dried cayenne and chipotle pepper for the basil and oregano).
Use a damp paper towel to wipe each mushroom clean, removing the stem if still attached. Use the tip of a spoon to gently scrape out the black gills. Cut each cap into 8 thin slices. In a shallow baking dish or baking sheet large enough to accommodate them all, lay out the mushrooms so that the slices form a single layer.
In a small bowl, whisk together the vinegar, olive oil, garlic, basil, oregano, salt, and pepper. Pour over the mushroom slices, tossing them gently to coat. Cover the mushrooms and refrigerate for at least 30 minutes (and up to 48 hours).
To assemble the sandwiches, spread 1½ teaspoons to 1 tablespoon of mustard (if using; or use hummus for more protein; see the tip below) on a slice of the bread. Lay 6 mushroom slices over the mustard, cover with a second piece of bread, and serve.