Next, make your cheese and onion sauce by melting the butter in a small saucepan and once melted, add the flour. Stir the flour through the melted butter for 2-3 minutes until the mixture has thickened and created a roux. Slowly add the milk to the roux to form the sauce. Once the sauce has a fairly thick texture add the cream cheese and mix well. Take the sauce off the heat and add the raw onion.