Bring a wide pot of water to a boil to cook the asparagus. Season prawns with a little salt and pepper. When the water is boiling, set a large skillet over medium-high heat. Add the olive oil and butter. When the butter has melted, add the prawns, lemon juice, garlic, cayenne and tarragon. Cook until the prawns are cooked through, about 3 to 4 minutes. (When done, the prawns will turn bright pink and be just slightly firm in texture.) While the prawns cook, set the asparagus in the boiling water and cook until just tender, about 2 minutes.
To serve, divide and arrange the asparagus on 4 plates. Divide and top with the prawns and the pan juices. Garnish with 1/2 lemon slices and serve.
Note: To devein the prawns, after peeling, use a paring knife to make a shallow slit.
Instead of prawns, use 20 large scallops in this recipe. Cooking time and technique remains the same. Or, use a mix of scallops and prawns in this recipe. Instead of asparagus, use 1/3- to 1/2-pound of green beans in this recipe. Cook and serve the beans as described for the asparagus.