1. In a medium bowl, whisk together lemon juice, lemon zest, red pepper flakes, salt, pepper, and honey.
2. While whisking, slowly drizzle in olive oil. Whisk until combined.
3. Add cucumber, fennel, and onion. Toss in dressing.
4. Add tomato and gently toss to combine. Check for seasoning and add more salt and pepper as needed.
5. Transfer to a serving dish and garnish with fennel fronds.