Remove stems and core the tomatoes. Cook tomatoes and jalapenos under the broiler, turning once, until black.
Peel garlic. Place in a small bowl with just enough water to cover and cook in the microwave one minute. Drain.
Remove jalapeno stems (and seeds and veins if you want milder salsa). Blend tomatoes (do not remove the skins), jalapenos, garlic, and salt, in a food processor until it is a very rough puree. Pulsing a few times should be sufficient. If you don't have a food processor, you can crush everything together with a molcajete.
Very finely chop the onion and cilantro, then thoroughly mix them into the tomato/jalapeno mixture. Add to jars and seal.