On adjusting recipes: I know you want to “make this your own,” but with canning recipes, you can only do so much. It’s important for food safety to have the proper ratio of acidic to non-acidic foods. The tomatoes are acidic, but the peppers, onions, and garlic are not. That’s why you must add the vinegar, and you can’t really mess with the amounts of peppers. You could, however, fiddle with green peppers and colored bells, or sub some of the jalapenos out for a milder pepper if you don’t like it so spicy. Just don’t be too generous with your helpings and overdo the amounts. That’s one thing I love about this recipe – it gives quantities in cups, rather than forcing me to scratch my head and wonder which onion is “small” and which green pepper fits the “medium” category. See this article on Modifying Canning Recipes and Food Safety for more details.
Preheat the broiler. Put the quartered tomatoes, sliced onion, and whole garlic cloves onto a roasting tray, spreading out evenly. Drizzle with plenty of olive oil and season well with salt and pepper and sprinkle with cilantro sprigs. Broil until everything is nicely charred, about 10 minutes (you want lots of deep rich color so don't be afraid if some of the edges get pretty black).
Now to the topic at hand – I’ve had the same concerns as you, especially since my dear husband is Mexican! We loved the canned salsa I made for the first week or two, then it was too vinegary, so now I use it for stuff like zucchini squash to use it up. Haven’t tried it again because, well we don’t have enough tomatoes yet and am leery about the vinegar and how to make it spicy enough. I never thought to skimp on the onions to compensate!!!
Have sterilized pint jars and lids and screw caps ready (they should all be washed in very hot water). Use a canning funnel and ladle hot salsa into jars, leaving a ½-inch head space. Wipe rims clean with a damp cloth and carefully place lid on and screw cap in place.  Process in a boiling water bath for 15 minutes, then place upright on counter for 24 hours (see recipe notes for link to USDA Canning Guidelines). You will hear popping sounds as the jars seal. If after 24 hours, any haven't sealed, put in refrigerator to use now.
I fell in love with Mexican food during a 2003 trip to California. Head over heels in love. I’m not talking your Chipotle Mexican Grill nonsense or your Taco Bell Tex Mex disgrace, I’m talking the sight and sounds of your host family’s Latin American sister-in-law chopping up wine-ripened tomatoes and fresh Jalapeños for Pico de Gallo on Thanksgiving. I’m talking pounding Coronas with Rudy all day and talking cultural differences, dreams and life in general. I’m talking the sound of Mariachi bands playing tableside in a side alley eatery near LA’s historic Olvera Street. The banging of pots from the stuffed taqueria kitchen. The smell of the spices hitting the heat of the stove… I love Mexican food. The authentic kind of Mexican food that few of us get to experience and shockingly many of us never knew existed… THIS, the authentic and the real, I love it! 
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