Cook prawns in a little water over low fire for 15 minutes.
Cut shell at the back and scoop out meat. set aside the shell and chop meat coarsely.
Saute onion in butter and lower heat then add flour. Stir well.
Remove from heat and gradually add milk, egg yolk (slightly beaten), and half of the grated cheese.
Season with salt, MSG, and calamansi juice. Lastly, add the prawn meat.
Continue cooking until the mixture thickens.
Fill prawn shell with cooked mixture, top with bread crumbs and the remaining grated cheese.
Bake in a preheated oven (375 °F) and bake for 20 to 30 minutes or until topping becomes golden brown.