Wash tomatoes, core, cut in half and place cut side up on large rimmed ungreased baking sheet.
Leave onion peeled, cut in fours, add to baking sheet skin sides down.
Peel garlic cloves, seal in a small piece of tin foil, add to baking sheet.
Bake until tomatoes and onion start to get soft, and brown a bit: about 20 to 30 minutes. Remove baking sheet from oven and let cool.
Heat an ungreased frying pan (e.g. cast iron or a griddle) until very hot.
Rinse dried chiles and pat dry.
Put the dried chiles in the pan, and let each side toast for about 10 seconds, or until they just start to puff. (Don't allow to scorch.)
Put them in a bowl and cover with the boiling water to soften -- about 15 to 20 minutes.
Remove peels on roasted veg and put them all into a food processor.
When the chiles are soft, remove the stems, rip them open and swish them in the water to rinse off the seeds. Add the chiles to food processor.
Add to food processor the vinegar, the cumin and the oregano.
Whiz the mixture in the food processor until smooth.
If desired, you may add a bit of the chile soaking water to thin it a bit. Discard the rest of the water, or freeze for another use such as cooking rice in. (Do not add more than a bit to the salsa, or you may lower its required safe acidity. Sorry, the recipe writers do not define what they mean by "a bit.")
If desired, adjust taste with salt (or salt sub) and pepper.
Put sauce in a pan and reheat on stove OR put in microwave-safe jug or bowl and zap until piping hot.
Ladle hot sauce into heated jars, leaving 2 cm (½ inch) headspace.
Debubble, adjust headspace.
Wipe jar rims.
Put lids on.
Process in a water bath or steam canner.
Process jars for 30 minutes; increase time as needed for your altitude.