Heat oil in a large frying pan or wok, over high heat. Add half the prawns and sear for about 45 seconds on each side until golden, then remove into bowl. Repeat and set prawns aside.
Steam mussels in a pot and heat up the pan. Add mussels and cover with a lid, cook for 4 minutes. Set aside.
Using the same frying pan or wok, heat sesame oil. Add garlic, ginger and chilli. Cook until garlic turns golden.
Add water, then Sriracha, soy sauce and sugar. Stir, increase heat to medium high, and let it simmer for 3 minutes until sauce is a thin syrup consistency.
Add prawns & mussels into pan, toss to coat in sauce and cook for 1 - 2 minutes until sauce is reduced and coats the prawns nicely.
Garnish with sesame seeds, fresh chilli and spring onions if desired. Serve with pasta, noodles or rice to soak up the awesome sauce.