Tear off 2 large pieces of plastic wrap (the type which is designed to use in the microwave oven); each one should be large enough to enclose the fish and vegetables in a single layer.
Rub lemon juice on both sides of halibut. Layer half cup of snow peas on bottom of plastic wrap and drizzle with a teaspoon of olive oil. Place fish over that and bring up plastic wrap to totally enclose the fish.
Set a cake rack over simmering water in a deep skillet. Turn fish packages upside down so snow peas are on top. Layer them on the cake rack and cover with a round bowl or cover to entrap steam. Cook for 5 minutes or until fish and vegetables are cooked through.
While fish is steaming, tomato seeds with a spoon and cut them into a fine dice; toss with dice with a bit of olive oil, salt and pepper and spoon the tomatoes on dinner plates.
When fish is done, with scissors, carefully open package (watch out for your hands). Pose fish and vegetables over tomatoes and drizzle juices over the top; season with salt and pepper and garnish top with basil.