With a heavy cleaver, chop each breast half into 3 pieces and each leg into 4 pieces.
Combine the oil, onion, and garlic in a 12- to 14-inch skillet or paella pan and place over low heat.
When the onion begins to sizzle, increase the heat to medium.
Add the chicken pieces a few at a time and cook until lightly browned, sprinkling them with a little of the salt, pepper and paprika as they cook.
As they brown, move the chicken pieces to the outside of the pan to make room for more in the center.
When all the chicken has lost its raw color, add the squid, peppers, green onion, and the remaining salt, and crumble in the saffron. Stir in the rice, cooking for a minute or so to coat it with the oil.
Add warm stock to within 1 inch of the rim of the pan (not all the stock may be needed).
Arrange the shrimp over the top, and the mussels or clams in a ring around the outside, hinged side down.
Cook at a lively simmer until the rice is just tender, about 20 minutes; scatter the peas over the top halfway through the cooking time.
The paella may seem a little soupy when it first comes off the fire, but the rice will absorb the excess liquid in a few minutes.
Tip:This works well woth boneless, skinless chicken thighs as suggested by YaYa.