Whisk together the lemon zest and juice, vinegar and some salt and pepper in a small bowl until the salt dissolves. Whisk in the honey, then slowly whisk in the oil until emulsified.
Combine the dandelion greens, fennel and onion in a large bowl. Drizzle some of the dressing around the sides of the bowl and then gently push and toss the greens to coat. Season with some salt and pepper and toss again gently. Serve immediately.