Preheat a large skillet over medium to medium-high.
Season the fish with salt and pepper and dredge in the flour. Add the oil to the skillet, then add the fish, presentation side down. Cook, partially covered, without turning, until the fish is browned and crispy, about 12 minutes.
Meanwhile, in another large skillet, melt the butter over medium-high. Add the leeks and peas and cook, swirling the skillet occasionally, about 5 minutes. Add the cream, reduce the heat a little and cook until the cream thickens and the leeks soften, about 5 minutes more; season. Add the juice of one lemon half.
Place 2 toast points on each plate. Top with the leek and pea mixture, then sprinkle with tarragon.
Turn the fish crispy side up and douse with the juice of the remaining lemon half. Set the fish on top of the leek and pea mixture on each plate.