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Leftover roast beef sandwich with easy homemade mayo as seen in this recipe.

Delicious Homemade Leftover Roast Beef Sandwich Recipe

This delicious homemade leftover roast beef sandwich recipe will show you the very best way to make the most of yesterday's Sunday lunch leftovers! We will help you to take that leftover roast beef sandwich and take it up a notch. We will also cover a delicious, easy mayonnaise recipe and a few handy tricks that make all the difference.
Prep Time 10 mins
Cook Time 5 mins
Course Lunch, Main Course, Snack
Cuisine English
Servings 1


Caramelised Onions

  • Butter
  • 1 Lrg Onion
  • 1 Star Anise Optional
  • Salt
  • 1 tsp Caster Sugar


  • 1 Egg
  • 1/2 tsp Mustard
  • 1 tsp White Wine Vinegar
  • 50 ml Neutral Oil sunflower, vegetable or non-virgin olive
  • Lemon Juice
  • Salt


  • Bread
  • Butter
  • Leftover Roast Beef
  • Lettuce
  • Mustard
  • Salt
  • Pepper


Caramelise the onions

  • Melt a good wedge of butter in a pan on low-medium heat.
  • Peel and slice up your onion. No need to dice it too finely. Add it to the pan.
  • Add the star anise as well, and a little salt.
  • Cook this, stirring frequently, for about half an hour, until the onion caramelises and appears almost to melt.
  • Remove the star anise and test the seasoning. Add more salt if necessary and if desired, add a little sugar.

Make the mayo

  • Crack the egg into a bowl and mix the yolk and white with an electric whisk.
  • Add a dash of vinegar and half a teaspoon of mustard and mix again, very thoroughly.
  • Add a tiny bit of the oil and mix.
  • Continue to add oil as you mix, a tiny bit at a time, until you've added the whole 50ml.
  • Season with salt and optionally lemon juice.

Build the sandwich

  • Cut two slices of bread and butter both sides of both slices.
  • In a dry pan on high heat, sear both sides until they turn a beautiful golden brown.
  • On the inside of one of those slices of bread, spread a thick layer of the mayonnaise.
  • On the other, spread a thin layer of mustard. Or a thick layer, depending on your mustard tolerance.
  • Finely, finely slice some lettuce and chop it up a little, then add it to the bottom piece of bread.
  • You can, if you like, season and dress the lettuce. I like to use a little salt and lemon juice.
  • Spoon the caramelised onion over the top of the lettuce.
  • Make a few very fine slices of beef, ideally somewhere in the middle, where the beef is still nice and pink. Fold them down gently into the sandwich. Try to keep them fairly loose.
  • Season the beef if necessary with a little salt and optionally a little pepper.
  • Put the sandwich together and slice it down the centre.
Keyword Leftover Beef, Leftover Beef Recipe, Leftover Beef Recipes