In a small bowl, crumble the saffron threads in the hot water; set aside.
In a large pan over medium-heat heat, heat the seafood or vegetable stock; reduce heat to low and keep broth hot (not boiling).
Seafood and Meat:
You will need a somewhat deep cooking dish, large cast-iron skillet, or the traditional Paella Pan (paellera), a round flat pan with two handles. The size of the dish or pan depends on the number of portions you wish to make. The amounts of the ingredients are relative and it does not matter if you use more of one and less of another or chose not to use some at all. The vital thing is the preparation of the rice so that it comes out loose.
In a large Paella Pan or heavy cast-iron skillet over medium-high heat, heat olive oil. Add the sausage rounds, moving and turning to get them evenly browned; remove from pan and set aside.
If needed add a bit more olive oil to the pan. Add the chicken cubes and stir-fry until done; remove from pan and set aside.
Add the prepared shrimp and sautuntil just done, approximately 2 minutes per side; remove from pan and set aside.
Reduce the heat to medium. To the same pan, add the onion or leek and garlic; saute approximately 5 minutes or until the onion softens. Add the tomato and pimenton or paprika. Cook, stirring often, approximately 15 to 20 minutes or until the mixture has darkened and thickened. NOTE: If the mixture start to stick to the bottom of the pan and burn, add some additional water.
Add the rice to the prepared Sofrito, stirring until well coated. Cook, stirring constantly for approximately 2 minutes. Pour 4 cups hot stock into the rice mixture. Add the saffron with its soaking water to the mixture. Use the remaining 1/2 cup of the hot stock to rinse out the saffron bowl and then add to the the rice mixture. Stir in the peas. NOTE: From this point on, do not stir the rice.
Simmer vigorously, moving the pan over 1 or 2 burners to distribute the heat and cook the rice as evenly as possible. After about 10 minutes, when the rice is at the same level as the liquid, reduce the heat to medium low. Arrange the meat and shrimp, pizza-style, over the top of the rice.
Continue to simmer gently approximately 10 minutes, rotating the pan as necessary, until all the liquid has been absorbed. Taste a grain of rice that is just below the top layer of rice. It should be al dente with a tiny white dot in the center. If the rice is not done, but all the liquid has been absorbed, add a little additional broth or water and cook a few minutes more.
To create the socarrat, turn the heat up to medium-high. Rotating the pan, cook for approximately 2 minutes or until the bottom layer of rice just starts to caramelize. NOTE: The rice will crackle, but if it start to smell burned, remove the pan from the heat immediately.
Finishing the Paella:
Remove the pan from the heat and cover loosely with a clean kitchen towel. Let rest for 5 minutes before serving. NOTE: Paella must rest after being removed from the oven, during which time the final cooking takes place.
To serve traditionally, set the paella in the middle of a round or square table so that everyone can eat it directly from the pan.
Additional delicious Paella recipes:
Paella with Shrimp
Spanish Mixed Paella
Paella with Spanish Rice - Rice Cooker Paella