In a large saucepan, melt the butter, add the flour and mix well. Gradually add warm milk in small batches the mixture will thicken slightly. Once you have added all of the milk, add the onion and potato and simmer for 5 minutes.
Add the tuna (including the juice from the can), corn and asparagus. Season with salt and pepper, to taste. Simmer for 5-8 mintues, stirring regularly.