- Preheat the oven to 190 ˚C (375 ˚F).
- Roll out the dough to make four 10-cm (4-inch) squares. Brush with milk and sprinkle with poppy seeds and onion salt. Bake in the middle of the oven for 15 minutes. Cool completely and slice each square in half horizontally.
- Mix the butter and flour. Set this mixture aside.
- In a saucepan, soften the shallots in remaining butter. On high heat, sauté the mushrooms until they are golden brown. Deglaze with the Pastis and reduce by half. Add the broth and cream. Simmer for 2 minutes. Add the reserved butter mixture and cook, stirring until it slightly thickens. Sandwich the mushroom sauce in the middle of each puff square. Sprinkle with tarragon.