You can prepare the dressing and chop the fennel and onion ahead; just keep the components separate until ready to serve.
Combine first 5 ingredients in a small bowl, stirring with a whisk. Combine fennel and onion in a large bowl; drizzle with dressing, tossing gently to coat.
Arrange 1 cup arugula on each of 8 plates. Top each serving with about 3/4 cup fennel mixture and 1 tablespoon cheese.
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