Bring a large pot of water to a boil for noodles. Salt boiling water and cook noodles to al dente.
While water comes to a boil, place oven rack in center of oven and turn on broiler. Heat a large skillet over medium heat with EVOO, a turn of the pan, and butter. When butter melts into oil, add mushrooms and sauté 5 minutes. Then add leeks and garlic and allow leeks to wilt, 5-6 minutes. Sprinkle flour over leeks and mushrooms and stir 1 minute. Then whisk in wine for half a minute, then whisk in milk and half and half or cream and simmer a couple of minute to thicken and reduce sauce. Then stir in Dijon mustard and season sauce with salt and pepper, to taste. Stir in peas, tarragon and tuna, then return sauce to bubble and turn off heat. Add cooked noodles to sauce, transfer to a casserole and top with Gruyere cheese. Pop casserole in oven and broil until bubbly and brown, 3-4 minutes. Garnish with chives or parsley and serve immediately.