Lay the asparagus spears in a roasting tin, drizzle with 2 tablespoons of the oil and toss until evenly coated in the oil. Roast in a preheated oven, 220°C (425°F), Gas Mark 7, for 15 minutes until tender and lightly charred in places.
Meanwhile, toast the sesame seeds in a dry frying pan over a medium heat, shaking the pan occasionally, for about 2 minutes until golden. Transfer to a jug with the tahini, lemon rind and juice, soy sauce, mirin and the measurement water. Add the remaining olive oil then use a stick blender to blend until smooth.
Spoon half the dressing over the roasted asparagus and toss well to coat. Arrange on 4 serving plates and serve the remaining dressing separately.