When canning mirepoix, I cook it all together first. So the onions, carrots and celery are diced, covered with water, brought to a boil and cooked for 5 minutes, before packing in 1/2 pint or pint jars with 1/2
tsp salt, then pressure canned for 40 minutes. I was told that when canning different ingredients, to use the most cautious technique, in this case, onions need to be cooked first and have the longest processing time. The mirepoix is totally cooked and soft, so it is best used in circumstances for which that is desired. I like to use it in quick soups, like broccoli cheddar, and braises, and just as a backup, if supplies are running low. I haven't tried canning trinity. I suspect that I won't like what it does to the peppers. Generally, I keep pepper strips in the freezer.