Dice mangos to your desired size. You can do large, medium, or small dice here. It's all preference and what type of bite you're looking for. Mince both the onions and garlic.
Add oil to pan, turn to medium and bring to temp. Add your onions, garlic, red pepper flakes and black pepper. Sauté until onions and garlic are translucent. About 3-4 minutes. If you're oil-free, use water or veg stock with a good non-stick pan.
Add mangos to the mix and cook for an additional 3-4 minutes.
Add all of your liquids, turn the stove on high and bring to a boil. Once boiling, add brown sugar and reduce it to a simmer. Simmer for 20-30 minutes until all liquid is gone and you're left with a thick sauce or jam-like consistency.
Spread on sandwiches or serve over seared tofu. If you eat meat, serve over chicken/turkey. Stays in the refrigerator for up to 1 week.