After sautéing for 3–5 minutes, stirring occasionally, the chopped onion should sweat. Heat medium or medium-low and stir frequently until chopped onions are golden brown, 30 minutes.
Sprinkle avocado oil on halved peppers in a single layer on a large baking sheet. Mix peppers with a little oil by hand. Roast 8–10 minutes to soften peppers without shattering structure.