Brussels Sprouts and Leek Gratin — a plant-based British recipe by BOSH!
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Brussels Sprouts and Leek Gratin

Honestly, Brussels sprouts get such a bad rep and this gratin is our answer to all of that. Creamy, bubbling, golden on top, it completely transforms two of the most underrated veg into something people actually fight over at the table. The leeks go silky and sweet, the sprouts get a little caramelised around the edges, and the whole thing comes together in this gorgeous, rich gratin that feels proper indulgent. It's the perfect side for a Sunday roast or a big festive spread, and it takes just 35 minutes. Make this once and you'll be making it every year.

Cook: 35 min
Serves 6

Before You Start

  • Preheat oven to 200°C
  • Large casserole dish
  • Frying pan
  • Microplane or fine grater (for garlic)

Ingredients

  • 2 tbsp olive oil
  • 500g Brussels sprouts
  • 2 leeks
  • 2 garlic cloves
  • 2 tbsp plant-based butter
  • 10g nutritional yeast
  • 1 tbsp Dijon mustard
  • 6 sprig of fresh thyme
  • 2 sprig of rosemary
  • 1 bay leaf
  • Salt & pepper to taste
  • 50g plain flour
  • 500 oat milk
  • 50g hazelnuts
  • 50g panko breadcrumbs
  • 1 tbsp nutritional yeast
  • Salt & pepper to taste

Method

1

Prepare the vegetables 

  • Trim and halve the brussels sprouts
  • Peel, trim, wash and slice the leeks
  • Peel and grate the garlic cloves
2

Cook the vegetables 

  • Warm the olive oil in the frying pan, add the leeks and garlic and stir for 2 minutes
  • Add the brussels sprouts and stir for 2 minutes
  • Spoon the leeks and sprouts into the casserole dish and smooth out across the pan with the back of a spoon
  • Set the pan to one side
3

Prepare the sauce 

  • Don’t clean out the frying pan
  • Add the butter to the pan and stir until it melted down
  • Add the flour to the pan and stir to combine
  • Gradually add the oat milk to the pan stirring constantly to make a nice smooth creamy sauce
  • Add the mustard and nutritional yeast into the pan and stir to combine
  • Add the rosemary, thyme and bay leaves to the pan, turn up the heat to a gentle simmer and let the flavour infuse into the sauce for 3-4 minutes
  • Fish out the rosemary, thyme and bay leaf, taste and season with a little salt and lots of pepper
4

Combine and roast 

  • Pour the sauce into the casserole dish and smooth out with the back of a spoon
  • Roughly chop the hazelnuts
  • Sprinkle the hazelnuts, panko bread crumbs and nutritional yeast over the contents of the dish
  • Drizzle over a little olive oil, put the casserole dish in the oven and roast for 20-25 minutes until the panko has turned a rich golden colour
  • Remove the tin from the oven and let it rest for a couple of minutes before spooning on to your Christmas dinner plates

Tips & Variations

  • Make it cheesier: We love loading the top with a good handful of nutritional yeast or a vegan parmesan-style cheese before it goes under the grill. Gets properly golden and adds a lovely savoury punch.
  • Swap the veg: If you can't get leeks, shallots work brilliantly here. They go even sweeter when cooked down and add a slightly more delicate flavour to the gratin.
  • Add some crunch: Blitz up a slice of stale bread and scatter the breadcrumbs over the top before baking. It gives you this amazing crispy crust that contrasts perfectly with the creamy filling underneath.

Why This Works

The trick here is giving the leeks and garlic a quick fry first. It draws out all that sweetness before anything goes in the oven. That little bit of colour on the veg before the sauce goes on is what gives you depth of flavour rather than just a bland, steamed result. Trust us on this one; don't skip that step.