In a large pot, heat 1 tablespoon oil over medium heat. Add 2 sliced leeks, 1 cup sliced cremini mushrooms and 1 bunch chopped asparagus; cook 4 minutes. Add 4 cups of water (or vegetable broth), 2 tablespoons fresh tarragon, 1 teaspoon cumin, salt and pepper to taste; simmer 20 minutes and puree. In a food processor, mix together 1 cup unsalted cashews and 2/3 cup water until smooth. Stir mixture and juice of ½ lemon into soup; heat 5 minutes.