Preheat oven to 350 degrees. Toast hazelnuts on a rimmed baking sheet for 8–10 minutes, tossing occasionally, until golden brown. Let cool, then roughly chop.
Using a mandoline or sharp chef’s knife, slice watermelon radish into paper thin rounds.
Pulse honey, garlic, oil and half of hazelnuts in a food processor to a coarse purée. Stir in vinegar, shallot, lemon zest and lemon juice; season with sea salt and pepper, to taste.
Using half the dressing, toss radicchio and radishes in a large bowl, then drizzle with remaining dressing. Serve topped with remaining chopped hazelnuts.