First let me say I love celery. I mean seriously love it. I keep slices in a refrigerator container and munch them like peanuts when I'm working. Radicchio is high on my list of likes as well. I put them together in this salad to take advantage of the best of many textures and flavors. Thin slices of sweet, crisp outer stalks of celery contrast with the tenderness of ultra-thin radicchio. Thin-sliced scallions deepen the flavors, while lemon and olive oil brighten them. Finally, you'll make a celery-leaf pesto to turn into the salad's dressing.
A note about Meyer lemons: they are naturally sweeter than conventional lemons. If Meyer lemons are not available, use one regular lemon and an orange or tangerine for both the juice and zest. —boulangere
Leave celery stalks whole. Hold each at an acute angle, and slice on thin setting of mandoline. If you don't have a mandoline, use a chef's knife held at an acute angle, and slice the celery as thin as possible. Transfer to salad bowl.
Slice each half of radicchio also on the thin setting. Leave shreds in bin of mandoline, and fill the bin with cold water. Let radicchio soak for 5 minutes. If you don't have a mandoline, shred radicchio using a knife and soak it in a mixing bowl. Soaking the radicchio for a few minutes will remove some of its natural bitterness. At end of that time, drain through a fine-mesh sieve. Transfer to salad bowl.
Slice scallions and add to salad bowl. Toss vegetables to blend.
Scatter about a teaspoon of coarse sea salt and some grinds of pepper over the salad. Squeeze lemon juice over, followed by 1/4 cup of olive oil. Toss to blend. Let sit for flavors to meld.
Place lemon zest, celery leaves, coarse sea salt, and garlic in a mortar and pestle. I like to use a wooden one to make pestos because the rougher surfaces generate good friction. Blend for about a minute. Set aside for 10 minutes for flavors to blend.
After rest period, whisk 1/4 cup of olive oil into the celery-lemon pesto. Use a spatula to scrape all of the dressing out onto the salad. Toss to blend.
Serve salad on chilled plates with additional wedges of lemon.