Bring salted water to the boil in a large saucepan.

Add the washed and sliced leeks and the Brussels sprouts, cover and let the vegetables cook for 4 minutes in the simmering water.

Drain the vegetables immediately.

In a saucepan over high heat, melt 30g of margarine.

Add the peeled and finely sliced garlic and let it cook for 1 minute.

Add the vegan bacon bits and let cook for 3 more minutes, stirring regularly.

Add 30g of flour and mix, then pour in the plant-based milk little by little while stirring constantly.

After a few minutes, once the mixture has thickened into a béchamel, add the mustard and the drained vegetables and mix.

Place the mixture into a medium-sized baking dish.

Preheat the oven to 190°C.

Roughly chop the hazelnuts and the rosemary (without its stem) for the crumble.

Cut the very cold margarine into small pieces.

In a mixing bowl, pour in all the crumble ingredients and season with salt.

Using your fingertips, quickly mix all the ingredients together until large crumble nuggets form.

Pour this crumble mixture over the gratin and bake for 20 minutes.