In a large cast-iron skillet or pan, heat the olive oil over medium heat. Sauté the chicken pieces for 3 to 4 minutes on each side until light golden brown. Remove from pan, drain on paper towels, or put in strainer to let the extra oil drip through. Set aside.
Turn the heat to low under the pan, add more olive oil if needed, add the garlic and onion, and sauté for about 5 minutes until the onion is brown on the edges. Stir occasionally.
Add green pepper, and cook for 3 minutes. Add the tomato, and cook for 3 to 4 minutes, stirring occasionally.
Add the parsley, salt, mashed saffron, peas, and olives, and stir gently to mix.
Stir in the rice and stock. Bring to a boil.
Reduce heat to simmer, cook for 15 minutes.
Add pieces of chicken, and optional shrimp. Continue cooking for another 15 to 20 minutes. Turn the shrimp over occasionally, or bury them deeper into the rice. Add more chicken stock as needed.
When the rice is soft and the liquid is absorbed, the paella is ready.