Gradually whisk in the chicken or vegetable stock and bring it to a gentle simmer. Let it cook for about 5 minutes until it thickens slightly.
Stir in the fresh lemon juice, seasoning with salt and pepper to taste. Then, whisk in the remaining 3 tablespoons of butter until the sauce is glossy and smooth. Mix in the chopped parsley.
Arrange the flounder on a serving platter, generously spoon the lemon butter sauce over the top, and garnish with additional parsley. Serve immediately with fresh lemon slices on the side if desired.
Notes
Perfect for a weeknight dinner or special occasion. Pairs well with salads or risotto.