Peel the asparagus and remove the woody parts. Cook in plenty of salted water with a pinch of sugar and 1 teaspoon of butter for just under 15 minutes. Lift out and set aside about 1 liter of asparagus stock. Cut the tips off the asparagus spears, set aside and mash the spears. For the risotto, finely dice the onion. In a wide saucepan, heat some olive oil and half of the butter and sauté the onions until light. Add the rice and stir until translucent. Pour in white wine, continue stirring and simmer. Gradually add the hot asparagus stock and simmer over medium heat, stirring constantly, until the rice is nice and creamy but still has bite. Just before the end of the cooking time, stir in the asparagus puree, the asparagus tips and the remaining butter. Season again to taste. Heat some olive oil in a pan and fry the shrimp briefly in it. Divide the risotto among 4 plates, arrange the crabs on top and sprinkle with chopped dill.