COOK MUSHROOMS AND ONIONS: In a large skillet (that has a lid), melt the butter over medium heat. Add the mushrooms and onion. Cook until the mushrooms begin to brown, about 7 minutes, and sprinkle with ½ a teaspoon of salt. Stir and transfer the mixture to a bowl.
COOK THE NOODLES: In the same skillet, add the noodles, milk, and broth to the pan and stir to evenly mix. Bring to a simmer on medium heat. Reduce the heat to low, cover and for 8-10 minutes or until the noodles are tender.
ADD CORNSTARCH: In a small bowl or ramekin, stir together the cornstarch and 2 tablespoon of water until the corn starch completely dissolves. Pour into the noodle mixture and stir to incorporate. Increase the heat to medium to simmer until the mixture thickens, about 5-7 minutes.
COMBINE REMAINING INGREDIENTS: Add the mushrooms and onions, the chicken and peas to the noodles. The sauce should be thick enough to just coat all the ingredients and have a little bit left in the pan. If there is a lot of sauce left, simmer a little longer to reduce.
ADD PARSLEY AND CHEESE: Remove from the heat and mix in the parsley and salt and pepper to taste. Top with cheddar cheese and bake in the preheated oven for 18 minutes or until the cheese is melted and bubbly.
SERVE AND STORE: Allow the casserole to cool for 5 minutes, then serve warm. Store leftovers in an airtight container for up to 5 days in the refrigerator.