Heat the oil in a large skillet over medium-high heat. Add the flour and cook, stirring often, until the roux is pale brown, about 10 minutes. Add the onion, celery, and bell pepper. Cook until the onion is soft and translucent, 3 to 5 minutes. Add the garlic and cook until fragrant, 45 to 60 seconds.

In small bowl, combine the tomato paste and stock. Add the stock mixture to the skillet and bring to a boil, stirring constantly. Decrease the heat to medium and simmer until slightly thickened and the flavors have married, about 10 minutes.

Add the shrimp, parsley, smoked paprika, and cayenne pepper. Cook until heated through, about 5 minutes. Taste and adjust for seasoning with salt and pepper. Spoon over hot rice, garnish with green onions, and serve immediately.