Instant Pot Instructions:

1. Use clean hands to combine the ground beef, bread crumbs, egg, diced onion, salt, pepper and half of tomato sauce in a mixing bowl. 
2. Pat the mixture into a pot-in-pot dish (I used this stainless steel pan*). 
3. In a small bowl stir together the rest of the tomato sauce, vinegar, brown sugar, mustard and Worcestershire. Spoon half of it over the top of the meatloaf.
4. Loosely cover the meatloaf with foil.
5. Pour 1 ½ cups water into bottom of Instant Pot. Place pan on a sling/trivet and lower the pan into the Instant Pot. 
6. Cover Instant Pot and secure the lid. Make sure valve is set to sealing. Set the manual/pressure cook button to 25 minutes. When time is up let pot sit for 10 minutes and then move valve to venting. Remove the lid.
7. Remove the pan and foil. Top with remaining sauce and put under oven’s broiler or air fryer lid for 3 minutes. 
8. Slice and serve. 

Slow Cooker Instructions:

1. Use clean hands to combine the ground beef, bread crumbs, egg, diced onion, salt, pepper and half of tomato sauce in a mixing bowl. 
2. Form the mixture into a loaf and place in the bottom of your slow cooker. 
3. In a small bowl stir together the rest of the tomato sauce, vinegar, brown sugar, mustard and Worcestershire. Spoon half of it over the top of the meatloaf.
4. Cover and cook on high for 3 hours or on low for 4-6 hours. 
5. Remove lid. Top with remaining sauce and put under oven’s broiler for 3 minutes. 
6. Slice and serve.