Adjust the oven rack to the middle position and preheat the oven to 475°F (245°C).
Heat 2 tsp of the coconut oil in a 10-inch nonstick skillet over medium heat until shimmering.
Add the scallion whites, celery, and bell pepper.
Cook until softened, about 5 minutes.
Stir in the garlic and 0.75 tsp Cajun seasoning and cook until fragrant, about 30 seconds.
Remove the skillet from the heat and stir in the 3 oz (85g) cheese until it is melted.
Transfer this mixture to a large bowl.
Gently stir the 12 oz (340g) crabmeat and 2 cups spinach into the bowl, being careful not to break up the lumps of crab.
Season with sea salt and pepper to taste.
Grease a 13x9-inch baking dish with coconut oil.
Pat the flounder fillets dry with paper towels and season them with sea salt and pepper.
Place the fillets, smooth side down, on a cutting board.
Mound the filling in the center of each fillet.
Fold the tail end over the filling, and then fold the thicker end over that.
Lay the flounder seam-side down in the prepared baking dish.
Toss the 0.25 cup breadcrumbs with the remaining 1 tsp of coconut oil, then sprinkle the breadcrumbs over the flounder.
Bake until the filling is hot and the flounder flakes apart when gently prodded with a paring knife, about 20 minutes.
Sprinkle with the reserved scallion greens and serve immediately.