Preheat oven to 400 degrees Fahrenheit. Grease a 9X13 casserole dish with butter or cooking spray.
Cook egg noodles according to instructions on box, minus one minute. Set a timer so the noodles are not overcooked. Once cooked, transfer to prepared casserole dish.
In a large stainless steel skillet, melt butter over medium-high heat. Add diced onions and saute until translucent and slightly golden, about 5-7 minutes, stirring regularly.
Add sliced mushrooms to the onions and continue cooking until tender. If mushrooms are of a thicker cut, reduce heat to medium-low and cook for longer so onions do not get burned.
Turn heat to low, add cream cheese, garlic, milk, and broth. Gently simmer, stirring often to break apart the cream cheese until thickened and combined well.
Add frozen peas and stir to combine.
Add flaked tuna chunks and grated cheese to the mixture and stir to combine.
Pour mixture over the cooked noodles and toss to combine with a pair of tongs. Season with salt and pepper to taste.
Sprinkle Parmesan cheese on top and bake uncovered for 15 minutes until bubbly and top is lightly browned. Serve hot.
To make this low carb/keto, omit liquid (milk + broth) and use cooked spaghetti squash or other favorite precooked aldente vegetable noodle. Reduce oven bake time to 10 minutes.
Leftovers can be stored in airtight container in the refrigerator for up to 4 days. To reheat on stovetop, transfer casserole into a small pot. Add just enough water to cover the bottom of pot. Close lid and reheat on medium heat until warmed through.
The information shown above is an estimate provided for your convenience by an online calculator. It should not be considered as a substitute for a professional nutritionist's advice. Please see our complete Nutritional Information Disclaimer.