Melt the butter (if using) in a small sauce pot and add the finely chopped onions. Cook over medium/low heat for 10-15 minutes, stirring occasionally. You want the onions to be very golden, almost paste like and just starting to brown.
Then add the red wine, if using, and cook for 2-3 minutes to reduce the wine by about half. Now add the broth, tamari, vinegar and all the seasoning and simmer for 10 minutes, whisking occasionally.
Meanwhile, make your cornstarch slurry by whisking together the corn starch and water in a small bowl. Mix well to ensure there are no lumps. Then pour the slurry into your gravy and simmer for 2-3 minutes to thicken the sauce.
Make sure to use LOW sodium vegetable broth. If you do not have low sodium broth, use only half the amount of tamari or soy sauce called for.Do not skip making the slurry. You don't want to dump cornstarch (or flour) into your simmering gravy or it will get lumpy.Make sure to use cold water to make your slurry (NOT hot). Then pour the slurry into your simmering gravy to prevent lumps.For picky eaters, you may puree your vegan onion gravy once cooked, to make it smooth with no onion pieces.*For oil free: omit the butter and use water or vegetable broth to cook down the onions. Make sure to add more as needed to prevent sticking.For soy free: use coconut aminos to sub for Tamari or soy sauce. Or just skip it add a little salt, to taste, instead.To make it corn free: sub the cornstarch for white or whole wheat flour, brown rice flour, or gluten free flour. But make sure to double up the amount since cornstarch has twice the thickening power of flour. (So use four tablespoons, not two.)For wine free: you may simply omit this step entirevly if you cannot consume wine or alcohol. No substitution is needed.Leftovers keep air tight in the fridge for up to 4 days, or freeze air tight (once cooled) for up to 3 months.