Preheat the oven to 350° F
In a large sauté pan over medium-high heat, add 1 Tbs. of butter and heat until foaming.
Stir in the mushrooms and cook them until they've given off all of their liquid and cooked through (~ 10 minutes).
Season lightly with salt and pepper, transfer to a bowl and set aside.
Add another ½ Tbsp butter to the frying pan, along with the onion, shallot, and celery. Saute for about 5 minutes, until they've softened.
Add the chopped herbs and sherry. Cook for another 2 minutes.
Stir the mushrooms into this mix and then set this aside.
In another saucepan, heat 3 Tbsp butter over medium-high, until foaming.
Stir in the flour to make a roux and cook for about 2 minutes.
Then whisk in the milk and chicken stock, bit by bit, making a smooth sauce.
Cook while stirring until just slightly thickened (another minute or two). Then add the lemon zest and season with salt and pepper to taste.
Flake the tuna and combine with the white sauce, mushroom-onion mixture, mixed veggies (if using), and the noodles
Grease an 8 X 8-inch baking pan and transfer the casserole mixture into it.
In a small pan, melt the last tablespoon of butter.
Stir in the minced garlic and the breadcrumbs. Cook, stirring until the breadcrumbs are golden brown.
Sprinkle them over the casserole.
Put the casserole in the oven and bake until it is bubbly (~ about 30 minutes)