Cook couscous according to package directions. Thaw shrimp if frozen. Preheat oven to 350 degrees F. Tear off four 15-inch squares parchment paper. Spoon about 1 cup couscous on one side of each sheet of parchment paper. Layer the broccolini, sweet pepper, and garlic next to the couscous. Top vegetables evenly with shrimp. Drizzle all with olive oil and balsamic vinegar. Top each with a piece of butter and sprinkle with salt and pepper. Add a sprig of oregano to each.
Fold parchment over shrimp and vegetables; fold in the open sides of parchment several times to secure. Divide packets among 2 shallow baking pans.
Place pans on separate oven racks. Bake about 20 minutes or until shrimp turn opaque. Open carefully to check doneness, as steam will escape. To serve, transfer packets to dinner plates. Top with prosciutto and, if desired, crushed red pepper.