Prep peppers: Cut the peppers in half and remove seeds and white membranes. Cut into ¼ inch strips.
Prep onion: Peel onion and slice thinly.
Heat pan: Place a skillet or frying pan over medium low heat. Add olive oil and once it’s hot and shimmering, add in and the onions. Cook, stirring, until soften (about 5 minutes).
Cook onions: Keep cooking another few minutes until they soften more, then add in the pepper slices. Stir to combine.
Cook onions and peppers: Cover with a lid, and let the peppers slowly simmer for about 15-20 minutes. Check on the peppers, and give them a stir so they’re not sticking. Add a little bit of water if they seem to be sticking, and stir, cover with the lid again.
Remove the lid and simmer: Take off the lid and keep slowly simmering. If you’re adding in tomato passata or sauce, add it in and stir to combine.
Add in salt: Season with salt. Give it another stir and if it’s sticking more and too thick, add a little bit more water, and stir, to loosen.
Keep simmering: Continue to simmer for another 10 minutes. The peppers should soften and be tender, but not broken down and mushy.
Check the seasoning: Add a bit more salt, if needed, to your taste.