Sauté the garlic and onions in olive oil on medium heat for 3 minutes
Add in the tomato concentrate and sauté for another 3 minutes
Add in all the prawn shells and increase the temperature to medium-high heat, sauté for another 20 minutes until the shells have taken on a really dark orange colour
Add in enough water to cover the shells and simmer for 30 minutes. Top up with water if necessary as it reduces. Blend all the shells into the liquid and immediately strain it through a fine sieve (important otherwise some shell bites might go through and your sauce will be very gritty)
Now strain the prawn stock on high heat until heavily reduced
Add in cooked 150g cooked pasta and season with Italian fish sauce (or salt if you can't find this), lemon juice to taste, and about 1 tbsp of diced butter
