- Crush the garlic; very finely chop spring onion; trim asparagus spears.
- For sauce: Grind walnuts along with onion, herbs, garlic and lemon peel. Whisk in the oil and then in the milk to form a smooth sauce of a fairly thick pouring consistency.
- With the oil, brush asparagus and then put under a preheated hot grill for about 6 minutes, turning frequently until golden and tender.
- In a gratin dish, transfer grilled asparagus and then pour over the sauce and then drizzle with the cheese. Return to the grill for about 2 minutes until bubbling.
- Serve immediately.