Floating Boxing Day Sandwich

French onion soup with a different set of priorities

After my birthday, today is my favourite day of the year. I’ve settled onto the sofa with a plate of leftovers balanced precariously on my lap, a child on each side and The Big Bang Theory on the screen (The Hawking Excitation episode in particular–you’ve got to watch it!).

Winter has felt particularly harsh this year. I’ve been travelling extensively for book promotion (UK, North America, Europe), followed by the run-up to a very busy Christmas in London. So I feel like I’ve earned my Boxing Day, this timely lull, and there’s room for indulgence and some rest.

Don’t get me wrong, I love my Christmas meal, but it's enveloped in too many expectations, the stakes are too high. The same food the next day, as leftovers, is less daunting—and I have to say it—more delicious.

My colleague, Jake, told me about a post-wedding breakfast in Porto, the kind that demanded something restorative. He found himself with a cold beer in hand and a Francesinha in front of him—a towering sandwich stuffed with steak, ham, and sausages, all smothered in a rich, bubbling beer-and-tomato sauce.

So, the Francesinha–a grilled sandwich swimming in a sauce–became the inspiration for a Christmas leftover sandwich.

Sauce-wise, we pivoted to something more in tune with the season: French onion soup, a bit like your gravy from the previous day, but lighter (BTW, your actual leftover gravy, with some caramelised onions folded through, would work just as well here; just loosen it slightly with some stock). It’s warming, with a comforting sweetness that feels just right for today. I think of it as a French onion soup in reverse–less of the soup, more of the cheesy bread.

Brussels sprouts—surprise heroes of the leftovers pile—make an appearance too. You do need something to cut through all that cheese, and the sprouts do this very well.

Our recipe is more suggestion than instruction—the beauty of a Boxing Day sandwich is in its flexibility. It’s about using what’s in your fridge: a leftover ladle of gravy, a splash of booze for a little extra depth, or the veg the kids didn’t end up eating (in my house, the dreaded parsnips, which are my absolute favourite).

So, happy Boxing Day–a celebration of second acts and softer edges. Eat, and then eat some more.

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Floating Boxing Day Sandwich

This recipe makes one large, hearty sandwich that could easily be shared between two people.

Prep 10 min
Cook 25 min

FOR THE FRENCH ONION SAUCE
1 large onion (180g), peeled and thinly sliced
25g unsalted butter
1 tbsp olive oil
1 tsp thyme leaves
1 garlic clove, crushed
½ tsp brown sugar
1 tsp plain flour
4 tbsp brandy or Calvados or cognac
350ml chicken stock (or vegetable)
½ tsp dijon mustard

FOR THE SANDWICH
60g cooked Brussels sprouts, shredded
80g gruyere, grated
2 slices of white sandwich loaf
20g mayonnaise
100g cooked turkey, sliced
2 slices of cooked ham or cured meat (25g)

1. Place a medium saucepan over a medium-high heat. Add the onion, butter, olive oil, thyme, ¼ teaspoon of salt and a generous grind of black pepper. Cook for 20 minutes until soft, jammy and caramelised, stirring frequently and turning the heat down to medium if the onions begin to catch.

   

   Add the garlic and sugar and cook for a minute, until the garlic is fragrant and turning golden. Stir in the flour and cook for another minute. Stir in the brandy and leave to reduce until it has almost all evaporated. Pour in the stock and mustard, then bring back to a simmer and leave to cook for two minutes more.

2. Meanwhile, preheat the oven to 180c.

3. Mix the sprouts and cheese in a small bowl.

4. Spread both slices of bread with mayonnaise. Place the turkey, ham and half of the sprout mixture on one slice of the bread and then top with the other slice of bread but this time with the mayonnaise side face up. Sprinkle the rest of the sprouts mix on the top.

   

5. Lightly press the sandwich and place on a parchment lined roasting tray and put in the oven for 10 minutes. Switch the oven to the grill setting and grill for 3-5 minutes until melting, bubbling and turning golden. Remove from the oven and place it on a plate with a lip. To serve, cut in half and douse with the piping hot onion sauce.