Pre-heat the oven to 200C/180C (fan)/Gas 6.
For that sauce, melt the butter inside a heavy-bottomed soup pot, add some flour and prepare gradually on the low warmth for several minutes, or before the mixture smells cozy. Pour within the chicken stock, show up the warmth and stir constantly until being applied. Add some cream and lightly simmer before the sauce until it features a syrupy consistency. Add some fresh lemon juice and tarragon. Switch off the warmth.
In another pan, lightly fry the chicken, all kinds of peppers and mushrooms inside a knob of butter for five minutes, then increase the sauce.
Butter an ovenproof dish big enough to carry the chicken cake mixture with room to spare. Add some chicken mixture towards the dish.
Unveil the pastry on the floured work surface, to some thickness of approximately ¼in/1cm. Put the pastry within the filling and thoroughly trim the perimeters. Make use of the trimmings to create cut-out shapes to brighten the top of cake. Cut a few air holes having a knife to permit the steam to flee. Brush the pastry using the beaten egg.