Start with a large saute pan that has recently fried meat and onions and the associated 1 tablespoon of grease/butter
[If you don't have meat remnants, add a small amount of salt]
add 1 tablespoon flour; stir until browned
slowly add 1 cup of warmed milk. Use the milk to scrape the pan clean
Add more warmed milk as needed to get the consistency
Re-add meat, sauted onions to gravy