Winner, winner, chicken-chorizo-shrimp dinner!
In a large nonstick skillet with a lid, melt the butter over medium-high heat. Add the garlic and tomato paste. Stir until aromatic, about 1 minute. Add the stock and saffron; season with salt and pepper. Bring to a boil, then stir in the peas and couscous. Cover and cook until the bottom begins to brown and get crispy (I check by lifting a corner with a spatula), 4 to 6 minutes. Remove from heat, add 1/2 cup water, and cover. Let stand until the couscous is tender, about 5 minutes.
In another large skillet, heat the oil, two turns of the pan, over medium-high. Add the chorizo. Cook, stirring often, until the fat begins to render, a minute or two. Add the shrimp and leek. Cook, stirring often, until the shrimp is opaque in the center, 3 to 5 minutes. Add the chicken and piquillos; toss to heat through, about 1 minute.
Using a spatula, turn sections of the couscous to expose some of the crispy bits. Arrange the chorizo, shrimp, and chicken on top. Squeeze the lemon half over the top. Sprinkle with the parsley. Serve directly from the skillet.