In a saucepan over medium to medium-low heat, add the stick of butter until melted then add the onion powder and whisk that in.
Add flour, a few tablespoons at a time, to the butter and whisk to form a roux or paste-like consistency.
Add about ¼ cup of stock, then a bit more flour and continue whisking to keep the roux. Next you will add the remainder of the stock and bring to a slow boil, whisking frequently.
Add salt and pepper to taste. Continue to boil for another 10 minutes or so until the gravy has thickened.