Lentil & Leek Gratin

📝 Ingredients

• 300 g dried green lentils
• 1 vegetable stock cube + 1 litre cold water
• 1 bay leaf + 1 tsp dried thyme
• 3 leeks (about 200 g each once trimmed)
• 2 garlic cloves
• 30 g unsalted butter (10 g for the leeks, 20 g for the béchamel)
• 40 g plain flour
• 600 ml semi-skimmed milk
• 2 tbsp Dijon mustard (30 g)
• 100 g grated Emmental
• Salt, black pepper, nutmeg

🍴 Utensils

• 2 saucepans
• Sauté pan with lid
• Whisk
• 20 × 30 cm baking dish
• Colander

📖 Steps

1. Rinse the lentils. Put them in a saucepan with the crumbled stock cube, 1 litre cold water, the bay leaf and the thyme. Bring to the boil, then cook at a gentle simmer for 20 to 25 min until tender but still whole, adding a little hot water if needed.
2. Meanwhile, trim the root ends and dark green tops off the leeks, halve them lengthways and rinse under running water, separating the layers to wash out any grit. Slice into 1 cm half-moons. Peel and chop the garlic.
3. Melt 10 g butter in a sauté pan over a medium-low heat. Add the leeks, garlic and a pinch of salt, cover and cook for 12 to 15 min, stirring occasionally, until meltingly soft but not coloured. Drain the lentils and remove the bay leaf.
4. Preheat the oven to 200 °C. For the béchamel, melt 20 g butter in the second saucepan over a medium heat, add the flour all at once and whisk for 1 min: the mixture forms a sandy paste that should cook without colouring.
5. Pour in the cold milk in three additions, whisking vigorously each time to prevent lumps. Cook for 5 to 6 min, whisking constantly, until the sauce thickens and coats the back of a spoon. Off the heat, add the mustard, nutmeg, salt and pepper.
6. Mix the lentils, leeks and a third of the béchamel in the sauté pan. Taste and adjust the seasoning, then spread the mixture in the baking dish in an even layer.
7. Cover with the rest of the béchamel, spreading it right to the edges, then sprinkle with the Emmental. Bake for 25 min, until golden on top and bubbling at the edges. Leave to rest for 5 min before serving: the gratin holds together better.