In a jar with a tight fitting lid combine the zest, juice, vinegar, mustard, agave, salt, and pepper. Close tightly and shake vigorously until combined. Store in the refrigerator until ready to use.
Make the pickled radishes
In a jar with a tight fitting lid combine the vinegar, sugar, and salt. Place the lid tightly on the jar and shake until the sugar and salt have completely dissolved. Set aside.
Use a mandolin to slice the watermelon radish in uniform slices that are paper thin. transfer the radish slices to the jar with the brine, seal tightly, shake to distribute the brine in between the radish slices, and transfer to the refrigerator for at least 10 minutes.
Build your salad
Spread the lettuces across a large platter, sprinkle with cucumber, radish slices,and fennel slices. Sprinkle the salad with the fennel fronds, pomegranate seeds, shredded green apple, and pepitas. Serve with the dressing on the side.
Alternatively, toss all of the ingredients into a large salad bowl. Pour 1/4 of the dressing over the salad and toss to combine. Serve immediately with the remaining dressing on the side.