Remove the shells from the shrimp (and heads, if still attached). If needed, devein the shrimp by cutting into the outside of the shrimp and removing the usually black string-like 'guts'. Store the reserved shrimp in the fridge until needed later.
To make the stock, place the shells (including heads, if you have them) in a pot and add the water. Cover, warm over a medium heat and bring to a boil. Reduce the heat slightly and let the mixture simmer for around 20 -30 minutes, preparing the rest as it simmers. Once the stock has cooked, strain to remove all of the shells.
Meanwhile, peel and finely dice the onion and garlic. Peel and dice the potatoes into relatively small, bite-sized pieces (you can leave skin on if you prefer). Cut the corn cob into slices around 1 inch (2.5cm) thick.
Towards the end of the stock cooking, or once it is done, add the oil in another pot over medium heat and add the onion. Cook until the onion is going translucent, around 5 minutes, but without letting them burn. Add the garlic and cook another minute.
Add the aji amarillo paste, tomato paste and oregano and stir through. Let it cook a minute. Add the strained stock and bring to a simmer.
Add the rice, corn slices and potatoes. Cover and simmer for around 10 minutes until the rice is cooked through and the potatoes are tender when tested with a knife.
Add the reserved raw shrimp, peas and chunks of queso fresco. Stir and push the shrimp under the liquid to ensure they cook through. Cook for another two minutes or so until the shrimp are cooked - they will go slightly pink and opaque. Add the evaporated milk, stir through to mix, then serve.