This requires a special piece of equipment, the paella pan. If you do not have one you can use a large cast iron skillet.
Heat olive oil and sauté onions until they start becoming translucent over a medium-high heat, about 3 minutes
Add garlic, chorizo and rice and mix and let it crisp up for a few
Cook for 5 minutes
Sprinkle saffron and add the stock to the mixture, stir
Add spices, tomatoes
Lower the heat to a low setting and let the paella simmer for 25 minutes covered. Stir.
Add the half cup of water
Add the shellfish, stir into rice and let simmer covered for fifteen more minutes.
Do not stir anymore, let it sit and cook
Add the chopped parsley on top, plate, and enjoy