Combine oil, lemon rind and juice, garlic, sugar, tahini plus salt and pepper in a large bowl. Whisk well to combine. Remove 1 tbs dressing and set aside. Add mushrooms to the bowl and stir to coat. Cover and refrigerate for 2 hours or until mushrooms have absorbed dressing.
Blanch, steam or microwave the asparagus until bright green and tender. Refresh in cold water, pat dry with paper towel and cut into thirds crossways.
Just before serving add asparagus, capsicum, watercress and almonds to the mushrooms and toss to combine. Arrange on a platter, drizzle with reserved dressing and toss gently.