Place the scallions and celery in a medium bowl of ice water. Soak until the vegetables curl, about 15 minutes.
Season the fish with salt and pepper. On a plate, mix the flour and dill. In a shallow bowl, whisk the eggs and 1 tbsp. lemon juice.
In a large skillet, heat 1 tbsp. EVOO, one turn of the pan, over medium-high. Melt the butter in the oil. When the foam subsides, dredge the fish in the flour, then coat in the egg mixture. Cook until the coating is golden and puffed and the fish is opaque in the center, 2 to 3 minutes per side. Transfer the fish to a plate.
Add the lemon slices to the skillet and cook until browned, about 2 minutes per side.
Drain the scallions and celery and pat dry. Wipe the bowl dry, then toss the scallions and celery with the lettuce and the remaining 2 tbsp. lemon juice and 2 tbsp. EVOO; season with celery salt and pepper. Divide the slaw among the baguette bottoms. Add fish, lemon slices and baguette tops.