Put the stock in a small saucepan over low heat. Put the saffron in a small bowl and add 1 tablespoon warm water; set aside.

Pat the chicken dry with a paper towel and season on both sides with salt and pepper. Put 4 tablespoons of the olive oil in a 9- to 10-inch paella pan over medium-high heat. When the oil is hot, add the chicken and cook until browned on the bottom, 5 to 7 minutes. Flip and cook until browned on the other side, another 5 to 7 minutes. Transfer to a plate and cover with aluminum foil.

Reduce the heat to medium. Add onion to the paella pan and cook, stirring occasionally, until it begins to soften, about 5 minutes. Add the garlic and cook until fragrant, about 30 seconds, stirring constantly to prevent burning. Add the tomatoes with their juices, sausage, and wine. Season with salt and pepper. Cook, stirring, until the mixture is reduced to a stew-like consistency, 4 to 5 minutes.

Add the squid rings and 1 tablespoon of the pimentón and stir well. Add the warmed stock and saffron with its soaking water and stir; bring the mixture to a simmer. Season to taste with salt and pepper. Add the rice in an even layer and press gently to submerge, but don’t stir—the pan will be quite full at this point. Cook the paella over medium high heat, without stirring, until much of the liquid is absorbed, about 10 minutes.

While the paella is cooking, toss the squid tentacles with the remaining 1 teaspoon pimentón and salt and pepper to taste in a medium bowl. Put the remaining 2 tablespoons olive oil in a small skillet over medium-high heat. Add the squid tentacles and cook, stirring occasionally, until just cooked through, about 2 to 3 minutes; the tentacles will curl and contract quickly in the pan. Remove from the heat, cover with foil and, set aside until ready to serve paella.

Gently nestle the chicken into the rice mixture (pour over any juices as well) and sprinkle the peas evenly over the top. Carefully cover the pan with foil, reduce the heat to medium-low, and cook until a meat thermometer inserted into the thickest part of one of the chicken pieces reads 165°F and all of the liquid has been absorbed, about 10 minutes. Remove the pan from heat and allow the paella to rest under the foil for 5 minutes or until ready to serve.

To serve, splash the paella with sherry vinegar and garnish with parsley, lemon slices, and the cooked squid tentacles. Serve with aioli, if desired.