For the seafood: Combine the olive oil, paprika, garlic, lemon zest, a pinch of salt and a few grinds of black pepper in a small bowl. Combine the frozen mussels and shrimp (still frozen) in a large bowl and drizzle the mixture over the top. Toss the seafood to evenly coat.
For the pouches: Lay four 16- to 20-inch-long pieces of nonstick aluminum foil out on a flat surface and top each one with a piece of parchment paper the same size. Divide the ingredients equally amongst each pouch, starting with the onions in the center of each in a 3-by-5-inch rectangle. Top the onions with the tomato halves, a pinch of salt, rice, peas, then seafood. To close, pull the long sides up over the mixture to meet and fold down together leaving some room for steam (about an inch above your pile), then close the short ends and fold upwards for easy opening.
Set the oven or grill to 375 degrees F.
Arrange the pouches on a baking sheet if using the oven, just in case you have a tiny spill. Bake or place the pouches over the indirect heat and cook for 25 minutes. Remove one and open gently; if the mussels are not open and the shrimp are still a bit underdone, close and continue to cook for 5 more minutes. Remove the pouches, and, when cool enough to handle, open and spritz with lemon and garnish with parsley.