In a slow cooker, combine chickpeas, water, and baking soda. Cook on high heat for 4 hours, or on low heat for 8-9 hours. Drain and cool. You will use 3-4 cups cooked chickpeas for this dish. Freeze the rest of the chickpeas in 8 oz. portions to have on hand.
Separate radish leaves from radish bulbs. Rinse radish leaves well, they tend to be sandy. Drain well. Rinse radish bulbs, cut into 1/8ths and set aside. Wash and drain mint leaves.
In the bowl of a food processor, combine radish leaves, mint leaves, toasted walnuts, parmesan cheese, garlic, salt and pepper. Pulse until all ingredients are chopped. With the motor running, add olive oil until just incorporated. Pour mixture into large bowl.
Chop scallions or thinly slice red onion.
Add 3-4 cups cooked, cooled, drained chickpeas, scallions or onion, and cut radishes to the bowl with pesto. Stir to combine. Adjust seasonings with salt and pepper. Sprinkle with sherry vinegar.
*You may also use two 15-ounce cans of chickpeas instead of cooking your own.