Pour 1cm of water into a large cold pan. Snap the woody ends off the asparagus, then place the spears in the pan and sit the fish fillets on top. Place on a high heat, cover and steam for 5 minutes, or until the fish is just cooked through. Meanwhile, trim the spring onions, roughly chop the green parts and place in a small pan on a high heat with half the lemon juice, 1 teaspoon of red wine vinegar, a pinch of sea salt and black pepper and 100ml of water. Bring to the boil while you pick half the tarragon leaves into a blender. Halve and destone the avocado, scoop the flesh into the blender, and add just the egg yolk. Pour in the hot spring onion mixture, then blitz until silky smooth and season to perfection. Very finely slice the whites of the spring onions and mix with the remaining tarragon leaves.Serve the fish and asparagus with the avocado hollandaise, finishing with pinches of the tarragon and spring onion and a light drizzle of extra virgin olive oil. Serve with lemon wedges, for squeezing over.