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Keep the temperature set to moderate-low, and add the diced onion to the same pan as the garlic and butter. Stir the ingredients regularly as they cook and as the onions become soft and semi-transparent. Use a wooden spoon to stir the onion and minced garlic. Reduce the heat of the stovetop down to the low setting. Stir the mixture continuously as it starts to simmer. Check the consistency of the gravy regularly, as the longer it simmers for the thicker it will become. The mixture will now be thickening due to the addition of cornstarch.

Summary:
Add the diced onion to the pan and cook it for a further 5 minutes. Simmer the ingredients for approximately 10 minutes.