FOR THE BASIL OIL
Blanch basil in medium saucepan of boiling water for 10 seconds. Drain. Rinse under cold water. Pat basil dry with paper towels.
Transfer the basil to a blender. Add the oil and puree until smooth.
Strain the oil if you want a refined look, but you can leave it as if you like.
(Can be made 3 days ahead. Cover and chill. Let stand at room temperature for 30 minutes before using.)
FOR THE NUAC CHAM SAUCE
Combine the lime juice, sugar and water, stirring to dissolve the sugar. Taste and adjust the flavors to balance out the sweet and sour.
Add the fish sauce and chili's. Taste again and adjust the flavors to your liking, balancing out the sour, sweet, salty and spicy.
FOR THE CEVICHE
Squeeze the limes into a bowl. Add the salmon and prawns. Stir and set aside.
Once the fish goes a little opaque in color a bit, drain the fish.
Add the onion and cilantro and stir.
Add a bit of the nuac cham sauce and stir. Add more to taste.
Plate the ceviche and then drizzle with the basil oil.
Serve with tortilla chips.