I've often made this and not had onion powder on the shelf, as a substitute I've reduced the number of garlic cloves to three and used the equivalent amount of garlic powder instead. You can also substitute the onion powder with 1/3rd of a medium onion, finely chopped which I recommend over the garlic powder.
If you forget about this sauce on the stove, which I have done on occasion, and then come back four or five hours later, you can revive it by adding in a few tablespoons of water, stirring gently to avoid the burnt bits on the bottom of the pan, and then let it cook for about 10 more minutes and then use or let it cool. Just avoid scraping the bottom of the pan and don't disturb the burnt leavings.
Can be used immediately after final seasoning, but a good simmering lets the flavour develop.
Stores in the refrigerator in a jar with a tight fitting lid for two or more weeks. Can be frozen almost indefinitely.
This recipe easily doubles. I often make up a quadruple batch once every couple of months and then store individual 120g (4 oz) portions in ZipLoc containers with snap on lids.
I use this sauce on my pizzas. I add it as a little extra flavouring to my ragu or simple sauce recipes. And we use it as the dipping sauce when I make all of my cheesy garlic breads.
I store this sauce in small Ziploc containers. Each container holds enough sauce for two 10" pizzas. When I need pizza sauce, I can either remove it from the freezer a few hours ahead of time, or nuke it in the microwave on low-power (pop the lid on the container) for a couple of minutes.