Preheat oven to 400 degrees Fahrenheit and line baking sheet with parchment paper.Spread brussels sprouts out on one or two baking sheets, add cooking fat and sprinkle with salt.Place in oven and roast for 50 minutes. Stir after 10 minutes to coat the melted fat evenly on the brussels sprouts.While the brussels sprouts cook, heat a cast iron pan over medium heat with more cooking fat (lard, duck fat, butter or avocado oil). Add onion slices and cook over medium-low heat for 20-30 minutes with ½ tsp salt. Stir occasionally to prevent too much sticking.Once the onions begin to brown and you get a sticky glaze on the bottom of the pan, use the balsamic vinegar to deglaze the pan.This means adding the vinegar and the steam produced will help release the glaze from the pan.Once everything is cooked, mix together and enjoy!