Whisk together lemon juice, olive oil, salt, and pepper in a medium bowl to make the dressing.

Boil salted water in a saucepan and cook shrimp until opaque, about 3 minutes.

Drain shrimp and rinse under cold water to stop cooking.

Drain shrimp well and pat dry, then halve lengthwise.

Peel grapefruits, removing all white pith.

Cut grapefruit sections between membranes over a bowl to release segments.

Drain grapefruit juice and reserve for other use.

Add shrimp, avocado, mango, and herbs to grapefruit sections.

Toss gently with dressing and remaining lemon juice.

Season with salt and pepper to taste.

Toss mesclun with remaining dressing.

Mound dressed mesclun on plates.

Spoon shrimp salad on top of mesclun and serve immediately.