Place the peppers on a grill or in the oven to roast until the peel is charred and the pepper is soft.
Transfer the peppers while hot to a clean plastic bag and close the top. This will create steam. Once the peppers have cooled, scrape off the peel, remove the seeds, and chop into 1/2 inch cubes.
In a small pot, heat the oil and add the peppers. Saute for 2 minutes and add the sugar, balsamic vinegar, and coriander seeds. Cook until the texture thickens to that of marmalade(around 5-7 minutes). Season lightly with salt, a shake of pepper, and a dash of chili flakes, if desired.
*If you have coriander seeds that aren't yet roasted, you can do it yourself. Place them in a small DRY pan(with NO oil) and heat on a low flame, stirring constantly, until fragrant. Add to the roasted peppers.