Bring the vegetable stock to a gentle simmer and keep it warm. Wearing gloves, wash the nettles well. Pick out any tough stalks or damaged leaves. Blanch the spinach and half the nettles, about 100g, in boiling salted water for 30 seconds. Lift them out and refresh briefly in cold water. Squeeze out the excess water, then blend to a smooth green purée with a little warm stock if needed. Set aside.
In a wide, heavy-based pan, warm the olive oil over a low to medium heat. Add the onion, celery and garlic with a pinch of salt. Cook slowly until soft and sweet, without letting them colour.
Add the rice and stir gently for 2 minutes, so each grain is coated and lightly toasted.
Pour in the white wine and let it bubble until the alcohol has evaporated. Begin adding the hot stock, one ladle at a time, just enough to cover the rice. Stir regularly, keeping the heat gentle. Let each addition of stock absorb before adding the next.
When the rice is almost al dente, stir in the nettle and spinach purée. Add the diced Robiola, the remaining raw nettles and the butter. Stir gently until the cheese and butter melt into the rice and the nettles soften.
Adjust the texture with a little more stock if needed. The risotto should be loose, creamy and flowing, not stiff. Taste and adjust the seasoning.
Spoon into warm bowls and finish with a generous grating of lemon zest.
Serve straight away.
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