In a pot make a sofrito, which is softening the onions, garlic and paprika (or chorizo) in olive oil, on slow heat, so they don’t burn. Once softened add the potatoes and fry them a little bit on the outside, then add the rice and keep on mixing everything so nothing burns. Add the water (you could add chicken stock as well) and salt. Mix a little bit so everything is incorporated, increase the heat and let come to a boil. Once the first boiling bubbles appear, reduce the heat and let simmer for about 15 minutes, check if the rice is cooked or if you need to add more liquid.
When the rice is done, turn off the heat and let it rest for a few minutes before serving.