1 medium portobello mushroom (at least 3 ounces), scrubbed clean and stem removed
1 tablespoon olive oil
Salt
Freshly ground black pepper
1 teaspoon balsamic vinegar
2 slices sourdough sandwich bread
1 clove garlic, halved lengthwise
2 tablespoons regular or vegan mayonnaise
1/2 cup packed baby spinach leaves
1/3 cup roasted red peppers
If you'd like, scrape the black gills from the underside of the mushroom and discard (they can be bitter). Cut the mushroom into 1/2-inch-thick slices.
Heat the oil in a medium frying pan over medium-high heat until shimmering. Add the mushroom and season with salt and pepper. Cook, moving rarely, until golden-brown all over, 5 to 6 minutes. Drizzle the vinegar over the mushrooms and stir to combine. Cook until the vinegar is completely absorbed into the mushrooms, about 1 minute. Remove the pan from the heat.
Toast the bread. Rub the cut side of the garlic on one side of each slice of bread. Spread a tablespoon of mayonnaise on each slice of bread. Place the spinach on one slice of bread, top with the mushrooms, and top the mushrooms with the red peppers. Place the other slice of bread mayonnaise-side down on top of the peppers to close the sandwich. Press down gently on the sandwich to compress and slice in half with a serrated knife before serving.
Recipes notes:
- Make ahead: If the mushrooms are made ahead, this sandwich can be put together at the last minute with just the aid of the toaster in the office. Pack the spinach and peppers separately. Toast the bread and proceed with assembling the sandwich.