Drizzle olive oil over the onion and season with salt and pepper. A drizzle of lime juice is a nice touch as well. Wrap the onion in foil and place in a 350 oven or indirect on grill for about 10 minutes.
Roast chilies. You can do this on your grill. We have a small chilie roaster and use the rotisserie.
When done place chilies in a plastic bag for about 10 minutes. Then peel, seed and chop. Chop onion.
Put the cut toms, (seasoned) rice wine vinegar and water in a small sauce pan and break up. I use a potato masher, works well. Add chopped chilies, onion, cumin and oregano. Bring to a boil then simmer just for a couple of minutes. DO NOT REDUCE.
After boiling let the salsa cool for about 20 minutes THEN add the cilantro. If you add it too early then the leaves cook and become wilted.