Heat a large nonstick skillet with a tight-fitting lid over medium heat. Add the EVOO, 2 turns of the pan, then add the onion and garlic and cook until softened, 3 to 4 minutes. Add the chorizo, increase the heat to medium-high and cook for 1 minute. Stir in the chicken stock, peas, piquillo peppers and saffron; bring to a boil. Add the chicken and shrimp and cook for 2 minutes.
Stir in the couscous, cover, turn off the heat and let steam for 5 minutes. Turn the heat back on to medium and cook until slightly crusty on the bottom, about 5 minutes. Stir to combine and top with the tomato and parsley.