In a large bowl, stir together 2 tablespoons olive oil, lemon juice, lemon zest, 2 minced garlic cloves, 1 tablespoon finely chopped parsley, ½ teaspoon salt and ¼ teaspoon black pepper. Add shrimp and turn to coat in marinade. Cover and let sit at room temperature for 30-60 minutes.
Heat remaining 1 tablespoon olive oil in a large pot over medium heat. Add onions and garlic, and cook, stirring, until onion is translucent and garlic is fragrant, about 3-4 minutes. Stir in brown rice, with remaining 1 teaspoon salt and remaining ¼ teaspoon black pepper.
Add vegetable stock and bring to a boil. Reduce the heat to medium-low, cover, and simmer for 40 minutes, or until water is absorbed and rice is tender.
Working quickly, spoon the shrimp in a single layer on top of the cooked rice and replace the lid. Cook until the shrimp is cooked through and turns opaque in color, 5 more minutes.
Serve warm with lemon wedges.