Slice the tips from the asparagus; blanch in boiling water for 1 minute. Drain. Coarsely chop remaining asparagus.
Heat oil and butter in a large saucepan over medium heat. Add leeks, onion and garlic; sauté until softened, 5 minutes. Add chopped asparagus. Cook, covered, 10 minutes.
Add rice, lemon zest, stock, salt and pepper. Partially cover; simmer for 30 minutes.
Purée soup in batches in a blender or food processor. Return to pan. Stir in tarragon and the reserved asparagus tips. Simmer 3 minutes. Remove from heat. Stir in yogurt and serve.