Melt butter, in a stainless steel or enameled cast iron pan, over medium heat.
Add flour and stir constantly with a flat-ended wooden spoon for 3-4 minutes. Make sure not to allow the mixture to burn.
Warm your milk and stock in the microwave or a separate sauce pan.
Remove your roux (butter/flour mixture) from the heat for just a moment. Start to slowly whisk in your milk and stock. Add your seasonings.
Do not get discouraged when this mixture first comes into a ball. Just keep adding your liquid and JUST KEEP WHISKING!
Move your sauce pan back to the heat and keep whisking. You may have to add additional milk to achieve the desired consistency. REMEMBER THIS NEEDS TO BE THICK, LIKE CONDENSED SOUP. This will ultimately help thicken your casseroles or side dishes.