Prepare the bell peppers by washing them and slicing into thin strips of about a quarter of an inch thick.
Slice the onions finely and mince the garlic cloves for additional flavor.
Heat oil in a large skillet over medium heat, then add in the sliced onions to coat with it.
Allow it to cook for 3-4 minutes, stirring occasionally until softened.
Add the peppers and garlic cloves, stirring to combine and cook for 5-7 minutes until the peppers are slightly softened.
Add red wine vinegar, granulated sugar, oregano, red pepper flakes, salt and pepper to the pan; stir to mix everything together and cook for an extra 3-4 minutes until the peppers are cooked through and lightly caramelized.
Enjoy this delicious Portillo's Sweet Peppers side dish with your favorite Italian dishes such as pasta, pizza, or risotto.