Loosen chunks of crab and roughly chop. Add pesto and 2 tsp (10 mL) lemon juice; toss until evenly coated.
Slice each avocado half into 8 slices. Arrange on a platter. Brush with remaining lemon juice. Arrange tomato and cucumber slices on platter. Mound crab salad in centre of platter; drizzle with dressing.