What's the secret?  Chef Missy fills us in

Steam cauliflower in a vegetable steamer over 2 inches of water, using a tightly-covered pot, for about 10 to 12 minutes until very tender. Alternatively, place cauliflower in a microwave-safe bowl , cover with water, and microwave on high for 8 to 10 minutes until very tender.

While waiting for the cauliflower to finish steaming, start to pulse the raw peeled zucchini with the lemon juice only (no water at this point). Drain the cooked cauliflower. Working in batches if necessary, add it to the pulsed zucchini in the bowl of the food processor with one tablespoon of water. Puree on high until smooth. Stop occasionally and push contents from the top to the bottom. If necessary, use the second tablespoon of water to make a smooth (but not wet) puree.

Makes about 2 cups of puree. Double recipe if you want to store even more, which can be done in the refrigerator for up to 3 days, or freeze 1/4 cup portions in sealed plastic bags or the small plastic containers.