Rinse the scallops under cold water to remove sand or impurities. Pat dry and slice into thin rounds or half-moons.

In a blender, combine tomatillos, jalapeños, key lime juice, onion, and cilantro. Blend until smooth.

Pour the marinade over the scallops and gently toss to coat evenly. Let marinate for about 10 minutes.

Prepare garnishes by slicing the cucumber and chopping extra cilantro. Arrange on a platter.

Spoon the marinated scallops into serving bowls, garnish with cucumber slices, cilantro, and jalapeño. Serve chilled.