1) Place flour in a shallow dish or re-sealable plastic bag and season with salt and pepper. Dredge fillets in flour mixture or add to bag, close, and shake to coat.
2) Place 15g of the butter and oil in a large frying pan over medium heat. When it foams, lower heat to medium-low. Shake excess flour off 2 of the fillets, add to pan, and cook until golden brown and crisp, about 2 minutes per side. Remove fillets to a plate and repeat with remaining fillets.
3) Add remaining 15g butter and cook until it begins to brown. Immediately remove from heat, stir in lemon juice, and pour over fish. Lay a lemon slice over each fillet, sprinkle pistachios over top and serve with mashed or roasted potatoes and a simple green salad.