Preheat your oven to 180 degrees celsius
Chop and slice your mushrooms, leeks and garlic
Add a knob of butter and a glug of oil to a casserole dish that can go in the oven later. Once your oil and butter has heated up, add your chicken thighs and move around the pan until they have sealed and started to brown.
Remove your chicken thighs and pop them on a plate. Add your leeks to your pan and sweat them down for a couple of minutes. Then add your mushrooms and garlic.
Pop your chicken back in the pan and add 175ml of white wine -reduce until it’s pretty much disappeared.
Add your tarragon and dijon mustard, coating everything in the pan.
Add your bone broth, collagen and season with salt and pepper.
Pop your pie in the oven and cook, uncovered, for 30 minutes or so. By letting it cook and reduce in the oven, you can avoid adding corn flour, which we think makes it gloopy and a bit meh. Give it a stir intermittently and keep an eye on how fast the sauce reduces - you want a bit left, but not so much that your chicken is swimming around.
Remove from the oven and add cream. You can use as little or as much as you want. 150ml should do the trick though. Taste to make sure it's seasoned to your liking.
Either keep it in one big casserole dish, or decant the filling into smaller pie dishes. Add your pastry or mash top and pop back in the oven until your mash starts to crisp, or your pastry is golden and flakey - 15 to 20 minutes should do it.
Serve with an array of greens - green beans, broccoli and cabbage are our fave.