In a saucepan over medium heat, melt butter. Add flour, salt and pepper, stirring constantly, until bubbly. Whisk in milk and keep whisking to break up lumps. Cook until thickened, about 1 minute. Add seasonings and herbs. Add tuna (including the tuna water in the can). Stir to break up tuna. Adjust seasonings. Add green onions at end. Pour over pasta or mix pasta in the tuna.

This is just a place to start. Add/subract stuff to taste.