Toss shrimp with garlic, fennel seeds, lemon zest, and salt in a bowl. Set aside.

Whisk together lemon juice, mustard, tarragon, salt, and pepper in a small bowl. Slowly whisk in olive oil.

Boil water in a medium pot, fill a bowl with ice water. Add shrimp to boiling water and simmer until opaque, about 30 to 90 seconds. Transfer to ice water to cool, then drain.

Combine cress, endive, celery, and chives in a large bowl. Drizzle with one-third of the dressing and toss. Divide among plates with avocado and shrimp; drizzle with remaining dressing.