Create the Cajun spice mixture by combining the flour, garlic, paprika, cumin, and chili powder in a bowl, mixing until all the spices are evenly distributed throughout.
Take the red snapper fillets and massage the spice mixture all over them, ensuring every surface is well coated. Cover the seasoned fillets and place them in the refrigerator to marinate for approximately 30 minutes, allowing the flavors to penetrate the fish.
When ready to cook, melt the 2 tablespoons of butter in a large frying pan over medium heat. Once the butter is foaming, carefully add the marinated fillets to the pan. Cook them gently for 2–3 minutes on each side, turning once, until the fish becomes opaque throughout and the flesh flakes apart easily when tested with a fork.
While the fish is cooking, prepare the cilantro butter: place the butter in a small bowl and let it soften to room temperature. Mix in the chopped fresh cilantro, a squeeze of lemon juice, and the ground coriander, blending everything together thoroughly. Season the butter with salt and black pepper to taste, then transfer it to the refrigerator to chill and firm up.
Once the fish is perfectly cooked, transfer the fillets to serving plates and top each one with a generous pat of the chilled cilantro butter, allowing it to melt over the hot fish. Add a garnish of fresh greens for color and serve immediately alongside fluffy rice.
