In a large mixing bowl, add pork, prawn, lemongrass, 1/2 of minced garlic, egg, and panko.
Season with 1 tbsp of fish sauce, salt & pepper to taste.
Mix well but do not over-handle meat as it will cause it to become tough.
Form into golf ball size meatball and steam for 8 mins on high heat.
Meanwhile prepare the sauce.
Heat pan on med high, add oil.
Saute onion and garlic until translucent.
Add tomato sauce, chicken stock, fish sauce, sugar, salt & pepper.
Cover & simmer until the tomato breaks down, about 10 mins.
Add the pork ball and all the broth in the plate.
Let it simmer or 5 mins in the sauce.
Stir in corn starch slurry to thicken sauce.
Remove from heat, and build your sandwich with parsley, carrot, meatball, sauce, and spring onions.