Cook the elbow macaroni according to package instructions until al dente. Drain and set aside.
In a large skillet, melt the butter over medium heat.
Add the chopped onion and cook for 3-4 minutes until softened.
Add the minced garlic and sliced mushrooms to the skillet and cook for another 4-5 minutes until mushrooms are tender.
Stir in the frozen peas and cook for 2 minutes until heated through.
In a medium bowl, whisk together the condensed cream of mushroom soup, whole milk, Dijon mustard, salt, and black pepper until smooth.
Add the cooked macaroni and drained tuna to the skillet with the vegetables and stir to combine.
Pour the soup mixture over the macaroni and tuna mixture and stir well to evenly coat everything.
Reduce heat to low and sprinkle the shredded cheddar cheese evenly over the top.
Cover the skillet with a lid and cook for 5 minutes until the cheese has melted.
Remove the lid and sprinkle the crushed potato chips evenly over the casserole.
Increase heat to medium and cook uncovered for an additional 3-4 minutes until the topping is slightly crispy.
Remove from heat and let the casserole rest for 2 minutes before serving.