Open steak sandwiches with dark fried onions and Béarnaise mayonnaise

Name: Open steak sandwiches with dark fried onions and Béarnaise mayonnaise
Prep time: 15 min
Cook time: 20 min
Total time: 35 min
Yield: 6
Ingredients: 4 sirloin steaks (not too thin, about 200g each); 2 tbsp groundnut oil; 2 onions, finely sliced; 6 slice/s rye bread; 50 g watercress

Angela Hartnett cooked this recipe by Diana Henry for Nick Grimshaw and guest David Harewood on episode 3, season 2 of Dish, the Waitrose podcast. It was served with these triple-cooked chips and paired with a whiskey sour cocktail and Elephant in the Room Pinot Noir Palatial.

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Serves6
CourseMain meal
Prepare15 mins
Cook20 mins
Total time35 mins
Plusresting

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4 sirloin steaks (not too thin, about 200g each)
2 tbsp groundnut oil
2 onions, finely sliced
6 slice/s rye bread
50g watercress

For the mayonnaise

1 British Blacktail Free Range Medium Egg yolk
½ tbsp Dijon mustard
175ml groundnut oil
1 tbsp white wine vinegar
1 shallot, finely chopped
1½ tbsp flat leaf parsley, finely chopped, plus 6 sprigs to serve
3 sprig/s tarragon, leaves only, finely chopped

To make the mayonnaise, mix the egg yolk and mustard in a bowl. Season. Using electric beaters, add the oil a little at a time, beating continually, only adding the next amount of oil when the previous one has been incorporated and the mixture has thickened. Do this until all the oil has been used. Add the white wine vinegar, shallot and herbs. Taste for seasoning and set aside.
Take the steaks out of the fridge 20 minutes before cooking and brush with oil on both sides. Meanwhile, add the rest of the oil to a large frying pan and fry the onions, first on a medium heat until soft, then on a higher one to crisp and darken them. It will take 12-15 minutes. Lift them out of the fat onto a double layer of kitchen paper. Sprinkle with a little salt. Transfer the onions to a bowl.
Re-heat the pan, plus another one so you have enough room for all 4 steaks. Once the pans are really hot, season the steaks and add them. Cook over a high heat, turning every 30 seconds or so, until they have a good brown crust. Each one will take about 3 minutes in total for rare (longer if using thick steaks). Remove to a plate, cover with foil and leave to rest for 5 minutes.
Spread some of the mayonnaise on each piece of bread. Remove the coarser stalks from the watercress and put sprigs on top of the bread. Slice the steaks, cutting on the diagonal. Divide the meat between the 6 slices of bread. Add some onions, but not so many that you can no longer see the pink meat. Spoon some of the mayonnaise on top, add a sprig of parsley and serve immediately.