Clean the asparagus by removing the wooden part. Cut the stalks into discs and leave the tips intact. In a pan, cook the asparagus with 2 or 3 tbsp oil and the stock, until they become soft. Keep the tips to the side and use a mixer to blend the rest together with the cheese and the milk, until you get a soft cream (if necessary, add some more milk or a spoonful of stock). Season with salt and pepper. Cook the penne pasta in plenty of water, drain and cover with the cheese sauce, carefully mixing. Add the asparagus tips kept to one side and serve.