In a large saucepan over high heat, bring the white wine and seafood stock to a simmer. Add a pinch of saffron threads and keep warm.
Meanwhile, put a large (16-inch) paella pan (see notes) over medium-high heat and add the olive oil. When hot, add the onion to the pan and saute until translucent, stirring often.
Add the garlic and cook until aromatic (about 1-2 minutes). Before it starts to brown, add the tomato and stir, adding in the paprika and a pinch of salt. Continue to stir and saute until the tomatoes are reduced and the oil is sizzling.
Gently add the squid to the pan and stir to coat in the tomato mixture. Saute for about a minute, to give some color to the squid.
Next, add the rice and stir to combine. After a minute of sauteing, spread the rice evenly across the pan (don't worry if it looks like a thin layer, it will puff up).
Gently add the simmering white wine and stock to the pan, all in one go. Reduce the heat to low, and cook, without stirring, for 10 minutes. Carefully lay the prawns/shrimp in a circle on top of the rice and cook for a further 10 minutes, turning the shrimp over as they start to turn pink. If the rice starts to dry out before the shrimp is cooked, you can top it up with a bit more hot stock, half a cup at a time.
Meanwhile, steam the mussels by putting them in a large skillet with a tight-fitting lid in half an inch of water. When the shells open, they are cooked! (Discard any that do not open).
When the stock in the paella pan fully evaporates, you will hear a faint cracking from the bottom of the pan as the socarrat starts to form. If you don't hear anything, you can raise the heat to medium for a short burst!
When the rice is ready, remove the pan from the heat and arrange the mussels on top of the rice. Serve from the pan, with wedges of fresh lemon.