*Chicken Mixture:

In large saucepan, heat oil over medium-high heat; saute carrots, mushrooms, onions, tarragon, salt u0026 pepper until onion is softened – about 8 minutes.

Add flour; cook, stirring, for 1 minute.

Gradually whisk in milk u0026 stock; bring to a boil, stirring.

Add beans.

Reduce heat u0026 simmer, stirring often, until thick enough to coat back of spoon, about 6 minutes.

Stir in chicken and mustard.

Spoon into round 8-cup (2L) baking dish; set aside.

(Make-ahead directions: Let cool for 30minutes, refrigerate, uncovered, until cold. Cover u0026 refrigerate for up to 2 days, rewarm in microwave to continue).

*Biscuit Topping:

In food processor, pulse together flour, baking powder and salt; pulse in butter until crumbly.

Drizzle in milk, pulsing just until soft sticky dough forms.

Turn out onto floured surface; knead 10 times or until smooth.

Pat out to fit top of baking dish.

Cut into 8 wedges; arrange over chicken mixture.

Bake on rimmed baking sheet in 400F (200C) oven until biscuits are golden and no longer doughy on bottom, about 40 minutes.