Add the diced onions and olive oil to a large pot with a lid (we used a Dutch Oven). Place over medium heat and cover with the lid. Cook undisturbed for about 15 minutes.
Remove the lid and stir the onions gently. There will likely be a lot of liquid in the pot, which is normal! Next, add in the balsamic vinegar, sugar, and fruit pectin/lemon juice. Stir to combine and reduce the heat to low-medium.
Allow to simmer without a lid for about 90 minutes, stirring once every 10-15 minutes to ensure the onions are not sticking.
Add small amounts of water at a time to help prevent sticking. We used about ½ a cup of water total, adding 2-3 tablespoons at a time.
Remove the jam from the heat once it has reached a dark brown color and the onions have a soft texture. Allow to complete cool before jarring. Store in the fridge.
