Preheat the oven to 350°F.
Bring a large pot of water to boil and cook the noodles according to package instructions, then drain and set aside.
While the noodles are cooking, add the butter to a large, heavy-bottomed pan or cast-iron over. Once butter is melted, add the onion, garlic, and mushrooms. Saute for 4-5 minutes, until the onion and mushrooms have started to brown.
Add the flour to the onion and mushroom mixture and toss to coat, then slowly whisk in the milk and broth. Continue whisking until there are no remaining clumps.
Next, stir in the salt, pepper, peas, and tuna and let the mixture come to a bubble. Once bubbling, add the cooked and drained pasta and toss to coat.
Transfer the mixture to a 3 quart casserole dish, top with cheese, if desired, and bake at 350°F for 20 minutes, until hot and bubbling. Remove from the oven, garnish with parsley, and serve!