Slice a red onion in half and remove the skin. Dice the onion into 1 inch pieces, leaving layers together. Dice the tomatoes into 1 inch pieces.
Add the diced tomato and onion to a bowl. Add the olive oil, oregano, salt, and red pepper flakes (optional) to the bowl. Stir to combine.
Add tomatoes and onions to air fryer basket. Cook for 12 minutes at 400 degrees F, tossing halfway through cooking.
Plate, and top with any fresh herbs if desired.
Notes
Don't worry if the red onion layers slightly separate while tossing with the oil and seasonings.
All air fryers cook differently. Check frequently to make sure tomatoes and onions are done to your specific liking.
I love to use air fryer parchment paper for easier cleanup (and I find it's easier to scoop up the tomatoes and onions), but it's absolutely up to you!