Brown the chicken in butter and season with salt and pepper.
Sauté mushrooms, onions, and garlic, then add the white wine.
Reduce for about 3 minutes on high heat.
Add the cream, both mustards, tarragon, and sugar.
Season to taste with salt and pepper.
Let the sauce simmer until it thickens.
Pour the sauce over the chicken.
Place the dish in the oven at 200°C for about 50 minutes.
Now spread the fresh spinach over the chicken and watch until the spinach "wilts" for about 4 minutes.
With a spoon, the spinach can now easily be mixed into the sauce between the chicken pieces.
Give the dish an additional 5-10 minutes before it's ready to serve.