Step 1:

Melt butter in a saucepan. Add flour and cook for 1-2 minutes.
Slowly add in some milk to the roux and whisk constantly (it will be super thick).

Step 2:

Keep adding a little at a time until it smooths out. Whisk until it reaches a boil and let bubble for 1 minute.

Step 3:

Add salt, chilli flakes, oregano, pepper powder (Be sure to adjust for taste)and continue to cook over medium heat, whisking continually until slightly thick. Finally, add cheese. Whisk until smooth.