Place almond meal, salt, and pepper in a large zip top bag. Add filets. Shake to coat.
Heat 2 tablespoons butter in a large skillet over medium heat. When it sizzles, reduce heat to medium-low. Shake excess flour off 2 filets and add to pan. Cook until golden brown and crisp, then turn. (Cook about 2 minutes per side.) Remove filets to a plate and repeat with remaining butter and remaining filets.
Add remaining 2 tablespoons butter and cook until it begins to brown. Remove from heat, stir in lemon juice, and pour over fish. Sprinkle with pistachios. Garnish with lemon slices, if desired.