Bring 2 cups shrimp stock to a boil in a saucepan; reduce heat to low and let the stock simmer.
Whisk cornmeal with remaining 1 cup shrimp stock in a bowl until smooth. Whisk the moistened cornmeal into the simmering shrimp stock. Stir grits occasionally until slightly thickened, 3 to 5 minutes. Stir in butter until incorporated. Mix in sharp Cheddar cheese, about 1 tablespoon at a time, until cheese has smoothly melted into grits.
Bring grits back to a simmer and cook, stirring often, until tender, 15 to 20 more minutes. Season with salt and black pepper.
While grits are simmering, heat canola oil in a large skillet over medium heat. Cook and stir celery until tender, 3 to 4 minutes; stir in onion, green bell pepper, thyme, and oregano. Cook and stir vegetables until onion is translucent, 7 to 8 minutes.
Mix tomato sauce into cooked vegetables and simmer for about 5 minutes to blend the flavors; stir in the sour cream to make a sauce.
Stir shrimp into the sauce with vegetables and cover skillet. Bring sauce to a simmer, cover, and reduce heat to low. Simmer until shrimp are pink and no longer translucent in the middle, 5 to 7 minutes.
To serve, turn grits into a large shallow serving bowl and arrange shrimp over the grits using tongs. Pour sauce and vegetables over shrimp and grits; season to taste with hot pepper sauce and sprinkle with green onion.