Once the bacon is done, transfer it to a plate with a paper towel to soak up excess grease, and then you will get started on your gravy in the same pan.
Take the aluminum foil you covered your frying pan with. Create a boat with the foil; this way, you can pour your bacon grease in it, not down your drain, and allow it to cool before throwing it away.
Add in the onion and turn on medium-low heat to start. Cook your onions in the bacon grease for 1 minute. Add in your minced cloves of garlic, and mix them into the onions.
Then add in your 2 tbsp flour and mix it into your onion and garlic mixture. Cooking for another 2 minutes, mixing occasionally so it does not burn. After pouring in your milk, start whisking it to thicken.