Heat the butter and EVOO in a large, deep skillet over medium to medium-high heat. Lightly brown the mushrooms, 10 to 12 minutes. Stir in the thyme, leeks, carrots and celery, and sprinkle with salt and pepper. Cook, partially covered, until the vegetables soften, 10 minutes. Sprinkle in the flour and stir 1 minute, and 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.
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.