Sounds tasty. I'd do something like - bottom filling quite saucy with chunks of chicken in a musroom/ tarragon/ white sauce, topped with an oaty crumble mix with some paxo mixed in... Might give that a go!
I've been shopping! Going to do chicken and leeks (and whatever left over veg is around) in white sauce with the topping like selks says - the crumble topping from the good food recipe with a bit of stuffing thrown in for good measure. I'll report back!
I used to make a vegetable version but haven't done in years...I'm definitely digging out my recipe. Haven't heard of a chicken version but sounds delicious. I am watching keenly for your report on his it went!