For the grits:
In a small pot over medium heat, add 2 cups chicken stock. Bring the stock to a boil, then stir in the grits. When this mixture starts to bubble again, season with salt and pepper, then lower the heat to a simmer and cook for about 4 minutes. When the grits begin to thicken, add the butter then turn off the heat, add the cheeses, stir until well combined, taste and season if needed.
For the shrimp:
In a mortar, crush together lime juice, garlic, coriander, 3 tablespoons oil, fish sauce, red pepper flakes, and salt and pepper until you from a paste, reserve 1/3 cup of this mixture in a small bowl. In a large sealable plastic bag combine shrimp with remaining mixture and marinate for 45 minutes at least in the fridge.
Drain shrimp and lightly pat dry between paper towels. In a large non-stick skillet pat 1 1/2 teaspoons oil over moderately high heat and sauté the shrimp until golden brown and cooked through, about 1 1/2 minutes on each side, then add the remaining sauce and turn the heat off. Place the shrimp over the grits and drizzle with the sauce and garnish with coriander sprigs. Serve immediately.