Prep for roasting. Preheat the oven to 425 degrees. Line a baking sheet with a piece of parchment paper.
Prep for canning. Prepare jars by washing them with hot soapy water. Place jars in the water bath canner fill the jars and water bath canner with water and place over high heat. Place the lids and rings in a saucepan and simmer over medium-low heat.
Roast veggies. Place tomatillos, onion, jalapenos, and garlic on the prepared baking sheet, and season with salt and pepper. Bake at 425 degrees F for 30 minutes or until the garlic has softened. Remove the garlic and then bake for 15 more minutes or until the onion, tomatillos, and peppers have slightly charred in areas. Remove from the baking sheet from the oven and allow to cool for 5 minutes.
Puree veggies. Once the peppers have cooled slightly, remove the stems and seeds. Place all of the roasted veggies in a food processor or high-speed blender. Working in batches if your food processor isn’t big enough process until the mixture is smooth.
Make salsa. Transfer the mixture to a saucepan and place over medium heat. Next, add the lime juice, cilantro, cumin, salt, pepper, and cayenne pepper, if using, stir until well combined. Bring mixture to a simmer and then remove from the heat.
Fill jars. Ladle the hot salsa into the hot jars leaving a ½ inch headspace. Remove air bubbles.
Wipe rims and fasten lids. Wipe jar rims with a clean moist cloth. Add the lids and rings to the jars, and tighten the rings so they’re fingertip-tight.
Process in canner. Transfer jars to the water bath canner. Make sure there is enough water so the jars are covered by at least 1 inch of water. Cover, and bring the water to a boil. Once the water starts boiling, process for 20 minutes, adjusting for altitude. After 20 minutes, turn off the heat and remove the lid from the canner.
Rest in canner before removing. Allow the jars to sit in the canner for 5 minutes. Remove the jars from the canner and place them in a heat-safe space.
Cool and check seals. Allow the salsa to cool for 12-24 hours. Check lids for a seal after 24 hours, once a proper seal has been confirmed, store jars in a cool dark place for up to 18 months or per your lids manufacturer’s recommendations.