Heat the olive oil in a pan that has a lid, add the onions and let them cook until just lightly golden brown over a medium high heat for about 4-5 minutes. Turn down the heat to medium low and let them cook another 2-3 minutes. Add the garlic and cook for 30 seconds. Quickly add the salt, cumin, coriander, turmeric and black pepper and cook another minute until the spices give off their scent. Add the tomato paste, stir so the spices are well incorporated into the tomato paste and cook for about 2 minutes, or until the tomato paste is a deep red. Add the chicken pieces and stir again until all the chicken is well coated in the tomato-onion mixture. Turn the heat up to medium and cook the chicken until it is no longer pink and is no longer completely raw. This will take about five minutes. Stir it frequently so all sides of the chicken pieces are cooking. Add the chopped tomatoes and continue to stir and cook until the tomatoes release their liquid. Add the water and then turn the heat down to medium low, cover the pan with its lid, and simmer gently for 15 minutes, stirring only occasionally to make sure nothing is sticking and there is enough liquid in the pan. After 15 minutes, remove the lid, stir the chicken and add the spinach. Let the spinach cook down and let the whole thing simmer again for 5 minutes, stirring frequently until the spinach is completely wilted and incorporated.