Melt butter in a medium saucepan, add bacon and onions then fry for two minutes. Add the flour and KEEN’S Mustard Powder and mix well with a wooden spoon for 1 minute. Add milk slowly and continue to stir until thickened.
Add diced fish fillets and tarragon into the sauce. Pour into an oven-proof dish. Top the dish with puff pastry and allow for shrinkage by having some extra overhanging. Cut a small cross in the centre of the pastry, brush with a little bit of excess milk.
Bake in the oven for approximately 20 minutes, or until pastry is golden and fish is cooked through.
Serve with steamed vegetables or a fresh garden salad.