Begin by caramelizing the onion, as it will require the most amount of time. Heat 1 tablespoon of avocado oil in a large skillet over medium-high heat. Add the chopped onion and sauté, stirring occasionally, until the onion begins to sweat, about 3 to 5 minutes. Reduce the heat to medium or medium-low and continue cooking, stirring occasionally, until the chopped onions have turned golden brown, about 30 minutes total. 

Pre-roast the sweet peppers while the onion is cooking (instructions below).

Once the onion is caramelized, add the garlic and spinach. Cover the skillet and cook for a few minutes to help steam the spinach. Stir well and continue cooking until the spinach has completely wilted and much of the liquid has evaporated.

Add in the feta cheese and stir well. Continue cooking and stirring until the cheese is mostly melted.

Preheat the oven to 350 degrees Fahrenheit.

Use a sharp knife to cut the tops off of the peppers, then slice them in half.

Transfer the halved peppers to a large baking sheet in a single layer and drizzle them with avocado oil. Toss the peppers with your hands to be sure all of the flesh is covered with a very light layer of oil. Roast for 8 to 10 minutes, or until the peppers have softened but still maintain their shape.

Remove the peppers from the oven and allow them to cool.

Once cool enough to handle, stuff the roasted peppers with the spinach feta stuffing and place them on a parchment-lined baking sheet.

Bake the stuffed peppers at 400 degrees F for 8 minutes. Place the oven on the High broil setting and broil for 2 to 3 minutes, or until the feta is showing a little golden-brown color.

For best results, serve the spinach and feta stuffed mini peppers fresh out of the oven. You can also make this as a make ahead appetizer and simply reheat the peppers in the oven when you're ready to serve.