Heat the butter and EVOO in a large, deep skillet over medium to medium-high heat. Lightly brown the mushrooms, 10-12 minutes. Stir in the thyme, leeks, carrot and celery and sprinkle with salt and pepper. Cook, partially covered, until the vegetables soften, 10 minutes. Sprinkle in the flour and stir for 1 minute, then pour in the wine, letting it absorb. Add the stock and bring to a bubble. Stir in the cream, season with a little nutmeg and let the sauce thicken a bit. Add the chicken and cook to heat through.
Bring a pot of salted water to a boil and cook the noodles to al dente. Stir the Dijon mustard into the creamy chicken, and then combine the sauce with the noodles and chopped herbs.
Cook’s Note: Cool and store the sauce for an easy make-ahead meal. Bring to room temperature before reheating over medium heat. Cook the noodles right before serving.