Preheat oven to 425 degrees F. Line a small baking sheet with foil, and fold up the edges all the way around so that the foil will trap any lime juice that escapes. Place a piece of parchment on top of the foil.
If using oil, rub the jalapenos and the lime halves with the teaspoon of cooking oil. Otherwise, just make sure the skins of the peppers are dry. Place peppers and limes on the baking sheet with limes cut-side down. Roast for 20 to 25 minutes, flipping the jalapenos at the halfway point. If too much juice seems to be dripping from the limes, flip them cut-side up. Once the pepper skins are charred, remove the tray from oven. If you haven't already, carefully flip limes cut-side up, so as to retain as much juice as possible. Let limes and peppers rest on the baking sheet for 10 minutes.
Once cool enough to handle, peel the skin off the peppers. Slice off the stem ends and cut the peppers lengthwise. Use the knife to scrape out the seeds. Discard stems and seeds.
*See Notes about using a blender instead of the food processor.In the bowl of a food processor combine the chickpeas, garlic, ¼ cup tahini, salt, 2 of the peppers, and juice from 3 of the lime halves. (The limes will be very soft, and some of the pulp should come out along with the juice.) Process hummus until mostly smooth, about 1 minute. Taste and adjust seasonings, adding a third jalapeno, juice from the remaining lime, extra tahini, and/or salt as desired. Process for another minute. If using oil, drizzle about 3 Tbsp olive oil through the feed tube while processor is running. For oil-free, drizzle in water until desired consistency is reached.
Store hummus in an airtight container in the refrigerator until ready to serve. If serving immediately, transfer hummus to a bowl, drizzle with a tablespoon or two of olive oil, and top with fresh cilantro. Optional garnish: if you have a roasted jalapeno remaining, chop it and sprinkle on top.
Leftover hummus can be stored in the refrigerator for up to 4 days.Nutrition calculations based on 6 servings, using ⅓ cup tahini and no olive oil.Making hummus in a high speed blender:Though hummus is often made with a food processor, you can use a blender for super creamy and fluffy hummus! You'll just need a bit more liquid to get the ingredients moving. Try using extra water or some of the aquafaba (liquid from a can of cooked chickpeas). To compensate for the extra liquid, taste and adjust seasonings as desired.