Use a pair of kitchen shears to cut the backs of the shrimp through the shell, and use a paring knife or skewer to remove the vein. Season the shrimp with 1 teaspoon of the kosher salt and 1/2 teaspoon of the pepper. In a mixing bowl, combine the olive oil, lemon juice, garlic, rosemary, marjoram, thyme, remaining 1 teaspoon of salt, and remaining 1/2 teaspoon of pepper. Reserve 6 tablespoons of the marinade separately. Place the shrimp in a large, re-sealable plastic bag and add the remaining marinade. Seal the bag, and massage the ingredients together to mix well, and place the bag in a casserole dish and then into the refrigerator. Allow the shrimp to marinate for up to 1 hour.

Set a grill to medium-high and remove the shrimp from the marinade. Wipe off the excess marinade and place the shrimp on the grill. Cook the shrimp until they begin to curl and turn pink, about 2 minutes. Turn the shrimp over and cook on the second side, another 2 to 3 minutes, until just cooked through. Serve the shrimp on a bed of couscous, drizzled with some of the reserved marinade.

To make the couscous, place the couscous in a stainless steel or glass bowl. Bring 1 1/4 cups of water to a boil in a saucepan along with the red onion. Pour the boiling water mixture over the couscous, wrap the bowl in plastic wrap, and set aside undisturbed until the couscous has absorbed the water and is tender, 5 to 6 minutes. Remove the plastic, fluff the couscous with a fork, add the parsley and season with salt and pepper, to taste.