Place a large pot of salted water over high heat and bring it to a boil.
Place a large skillet over medium-heat with the EVOO and butter. Add the mushrooms to the pan and cook until golden brown, 6-7 minutes.
Add the onions and cook until tender, 5-6 minutes. Sprinkle the flour into the pan, stir it into the veggies and cook about 30 seconds. 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 freshly ground black pepper, and cook another minute until it has thickened up. Don't worry if it's a little loose, we'll be adding more stuff to it later on.
With your veggies cooked and the sauce working, drop the egg noodles into the boiling water and cook them to al dente according to package directions. While the noodles are cooking, finish off your sauce by tossing in the tarragon, tuna or chicken, and peas. Give everything a good stir and simmer over medium heat until the peas and meat are warmed through, about 3-4 minutes.
While the filling and noodles are cooking, toast and butter your English muffins, then toss them into a food processor and grind them up into coarse bread crumbs.
Once the noodles are cooked, drain and toss them back into the pot you cooked them in. Toss in the sauce and veggies, and give everything a good stir to combine. Scoop the filling into serving bowls and top each with some of the English muffin breadcrumbs.