Shaved Portobello Mushroom Sandwiches

Portobello Mushroom Sandwiches

Shaved Portobello Sandwiches with Pan Roasted Onions and Black Pepper Mayo

Here’s a vegan take on the cold roast beef sandwich that as satisfying as the original. There is a little work involved to put all of the components together, but it can all be done ahead of time and assembled when you’d like.  Sliced, chilled pan roasted Portobello mushroom caps are remarkably full flavored and make a great sandwich filling on their own, but when you add some roasted onions and a little of this seasoned mayonnaise to the mix, the flavors go in to overdrive.  The secret is to take some of that rich glaze leftover from pan roasting your mushrooms and add it to the mayo.  If you don’t have the glaze you can make the mayo without it, but you won’t believe the flavor difference between the two. This is just the kind of thing to share with your omnivore friends, to show them that we eat way more than just salads!

Pan Roasted Portobello

Concentrated Mushroom Jus

Sliced Portobello

Portobello Sandwhich

Shaved Portobello Mushroom Sandwiches
Prep time
Cook time
Total time
A Vegan take on the cold roast beef sandwich, this pan roasted Portobello mushroom sandwich comes with roasted onions and an awesome horseradish mayo
Serves: 1 sandwich
Per Sandwich:
  • 1 Ciabatta bun, split horizontally in two
  • 2-3 tablespoons Black Pepper-Horseradish Mayo (see below)
  • A small handful of baby spinach, tossed in a ¼ teaspoon olive oil and a tiny squeeze of lemon juice, salt and pepper to taste. Massage until lightly wilted.
  • 2-4 tablespoons prepared Pan Roasted Onions
  • 1 Pan Roasted Portobello mushroom, chilled and very thinly sliced on an angle. Think shaved roast beef as a guide
Black Pepper-Horseradish Mayo
  • Makes around ¾ cup, or enough for 4 sandwiches
  • 8 tablespoons/ ½ cup Vegan Mayo
  • 3 tablespoons concentrated mushroom drippings from a recipe ofPan Roasted Mushrooms (sorry, there is no substitute)
  • 1 tablespoon drained prepared horseradish
  • 1 ½ teaspoons Dijon mustard
  • ¾ teaspoon cracked black pepper
To Make the Black Pepper-Horseradish Mayo
  1. Combine everything in a bowl. It will stay fresh for around a week in the refrigerator
To Complete the Sandwich
  1. Toast the ciabatta halves in a hot, dry pan or under the boiler, until well toasted
  2. Spread both sides with a generous amount of the black pepper mayo, at least 2 tablespoons
  3. On the bottom half of the bread layer on some of the massaged baby spinach. Top with some roasted onions, then layer on the finely sliced Portobello mushrooms. Top with the other half of the ciabatta

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    Pan Roasted Red Onions - Two Happy Rabbits
    September 12, 2017 at 5:11 pm

    […] So, that’s what we’re doing here with these onions. The rings are allowed to stay in contact with the moderately hot pan until they brown on one side, then they are flipped or mixed and a new set of onion surfaces can be in contact with the pan. The flipping and the roasting continue until the onions are suitably cooked and browned. The fresh thyme, while it adds a nice background flavor to the onions, could be replaced by dried thyme (a bit more than ¼ teaspoon), or even omitted all together. Try these as part of our Shaved Portobello Sandwich with Roasted Onions and Black Pepper-Horseradish Mayo […]

  • Reply
    Pan Roasted Portobello Mushrooms - Two Happy Rabbits
    September 12, 2017 at 5:12 pm

    […] They can be served hot, right from the pan, or chilled and used later. Once cold, they can be thinly sliced and used as a chilled ‘roast beef’ substitute. Split them in half horizontally and stuff with your favorite non-dairy cheese and bake (or bread and fry!) until hot. Or simply toss with a little oil and reheat them on the grill for the moistest mushroom burger you’ll find. Try these as part of our Shaved Portobello Sandwich with Roasted Onions and Black Pepper-Horseradish Mayo […]

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