Place shrimp in a large bowl of cold water until thawed, 10 to 15 minutes.
Melt 1/2 cup butter in a microwave-safe glass or ceramic bowl in the microwave in 15-second intervals, 30 seconds to 1 minute. Mix in garlic and half the lemon juice.
Melt 1 1/2 teaspoon butter in a large skillet over medium heat. Add onion; cook and stir until translucent, about 5 minutes. Add shrimp, garlic butter mixture, and sambal oelek. Cook and stir until garlic is cooked and shrimp are heated through, 7 to 8 minutes.
Bring water and sushi rice to a boil in a saucepan. Reduce heat to low, cover, and simmer until rice is tender, about 15 minutes. Remove from heat, and let stand, covered, until liquid has been absorbed, about 10 minutes.
Serve shrimp over rice, topped with remaining lemon juice and green onions.