Heat oil in pan to medium.
When oil is shimmering, add onions. Stir to coat with the oil and spread out so all the onions have contact with the pan.
Stir onions occasionally (about every few minutes) and reduce heat to prevent burning, if needed. Try to leave them alone enough to brown, but not so long so that they burn.
Deglaze the pan with the white wine when the onions start to stick to the pan.
Onions will need to cook 30 minutes to an hour to caramelize. You may optionally add a little sugar to speed up the caramelization process if you are in a hurry and don't mind the added sugar.
Meanwhile, steam the green beans and peppers. You may do this traditionally or in a pressure cooker. Pressure Cooker instructions: Pour a cup of water into an Instant Pot, add a steam basket, and then add all of the green beans and cut peppers. Seal and set manual pressure to 0 minutes.
When both caramelized onions and steamed green beans and peppers are complete, mix them both together and season with salt and pepper to taste. Enjoy as a fancy side or healthy alternative to Green Bean Casserole at Thanksgiving dinner!