Place a large pot of salted water over high heat and bring to a boil.
Place a large skillet over medium-high heat with EVOO and butter. Add the mushrooms to the pan and cook until golden brown, 6-7 min.
Add the onions and cook until tender, 5-6 minute Sprinkle the flour into the pan, stir it into the veggies and cook about 30 sec. Slowly whisk the chicken stock, milk and dijon mustard into the pan, letting each liquid thicken up before adding the next. Season with salt and pepper, and cook another minute until it has thickened up.
With the veggies cooked and the sauce working, drop the egg noodles into the pot with the salted water and cook them to al dente according to package directions. While the noodles are cooking, finish off the sauce by tossing in the tarragon and chicken. Give everything a good stir and simmer over medium heat until the chicken is warmed through, about 3-4 minute.
Once the noodles are cooked, drain and toss them back into the pot they cooked inches Toss in the sauce and veggies over the noodles and give everything a good stir to combine. Top with bread crumbs.