Me again. I heard from the recipe author this morning. She stated that she uses one of the on-plan ketchup recipes from the Condiments forum (she didn't tell me which one) and basically uses all of it. As for the brown sugar substitute quantity (and puh-LEEZE be sure not to buy Brown Sugar Splenda because it contains real sugar) she states she likes it really sweet, so she uses the equivalent of about 1/2 cup.
Since we all have different tastes, you may wish to start out with much less than that, stir it in and let it cook for a bit, then taste to see if you wan to add more before proceeding. (Also worth noting, some of our on-plan ketchup recipes may be sweeter than others.
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Paula also stressed that she feels this tastes much better if it's left overnight in the fridge before serving-- that the extra time soaking up the flavor of the onions, ketchup and brown sugar really makes the flavor even better.