Prepare the chicken
Pat the chicken thighs dry.
Season both sides with salt and black pepper.
If some thighs are substantially thicker than others, flatten them slightly so they cook at roughly the same rate.
Brown the chicken
Heat a large frying pan over medium-high heat.
Add the olive oil.
Place the chicken into the hot pan without overcrowding it and cook for approximately 4–5 minutes on the first side without constantly moving it.
Turn and cook for another 3 minutes.
You're mainly developing colour at this stage. The chicken doesn't need to be completely cooked through.
Transfer to a plate.
Brown the mushrooms
Check how much fat is left in the pan.
You want enough to lightly coat the mushrooms, but not a large pool. Remove excess chicken fat if necessary.
Increase the heat slightly and add the mushrooms.
Spread them across the pan and leave them alone for 2–3 minutes before tossing.
Continue cooking for another 3–4 minutes until well browned.
This is one part I wouldn't rush. Pale, steamed mushrooms and properly browned mushrooms produce very different sauces.
Add the aromatics
Reduce the heat to medium.
Add the shallot and thyme and cook for around 2 minutes.
Add the garlic and cook for another 30 seconds.
Don't let the garlic burn.
Deglaze with white wine
Pour in the 150 ml white wine.
It should immediately begin bubbling.
Use a wooden spoon to scrape the browned chicken and mushroom bits from the bottom of the pan.
That's all going into the sauce.
Let the wine simmer uncovered until reduced by roughly half, about 3–5 minutes depending on the size of your pan.
Don't rush straight into adding the stock.
I want the wine to concentrate first.
Build the sauce
Add the chicken stock and Dijon mustard.
Stir until combined and bring back to a gentle simmer.
Return the chicken and any juices collected on the plate to the pan.
Finish cooking the chicken
Simmer gently for approximately 5–8 minutes, depending on the thickness of the thighs, until the chicken is cooked through.
If the sauce is reducing too quickly, lower the heat.
If the chicken finishes before the sauce reaches the consistency you want, remove the chicken and keep it warm while reducing the sauce for another minute or two.
Finish with butter
Remove the chicken again or push it towards the edge of the pan.
Take the pan off the heat.
Add the 20 g cold butter and swirl or stir it through the sauce until completely incorporated.
This gives the sauce gloss and richness without needing cream.
Taste.
Only add the lemon juice if the sauce actually needs more acidity. Depending on the wine you've used, it may already have plenty.
Adjust salt and black pepper.
Serve
Spoon mashed potato onto the plate.
Add the chicken and green beans.
Spoon the mushrooms generously around and over the chicken, followed by the white wine pan sauce.
Finish with chopped parsley and a little freshly cracked black pepper.