In a blender or food processor, combine the avocado flesh, parsley, chives, tarragon, grated garlic, lemon juice, olive oil, water, salt, and pepper. Blend until completely smooth, stopping to scrape down the sides once or twice. The dressing should be thick but pourable - add another tablespoon of water if it seems too stiff. Taste and adjust salt or lemon as needed. Transfer to a jar and set aside.

Tear the lettuce leaves into bite-sized pieces if they are large, or leave small leaves whole. Pat completely dry with a clean kitchen towel - wet lettuce will dilute the dressing. Slice the cucumber into thin rounds or half-moons. Halve the second avocado, remove the pit, and dice the flesh into half-inch cubes. Slice the radishes paper-thin using a mandoline or sharp knife.

Place the dried lettuce in a large bowl. Add the cucumber and radishes. Spoon about two-thirds of the dressing over the greens and toss gently with your hands or salad tongs until everything is lightly coated - you want the leaves glossy, not drenched. Tuck the diced avocado throughout the salad so it doesn't break apart. Scatter the toasted pepitas over top. Serve right away with extra dressing on the side.