In this case you can replace the ricotta mixture with a bechamel. Melt 1/4 cup butter in a large pot over medium heat. Add the flour and let it bubble for about 1 minute, while whisking to ensure everything is smooth. Add 1 cup of milk and whisk until completely smooth, then whisk in another 2 1/2 cups of milk. Finally, stir in 1 cup of freshly shredded parmesan cheese and taste for salt and pepper.