Heat 2 Tablespoons of butter in the empty pot over medium high heat. Add chopped onion and season with salt and pepper. Sauté onions until translucent and soft. Add garlic and cook for 1 minute. Add remaining tablespoon of butter and flour. Stir and cook for 1 minute. Whisk in chicken broth and a pinch more salt. Stir and bring to a boil until thickened, about 3 minutes.
Turn off the heat. Add cream cheese and cheddar and whisk until melted. Add peas and allow them to warm through. Taste cheese sauce and add more salt and pepper if it needs it. Stir in the drained noodles and tuna. Pour mixture into 9 x 13 pan or casserole dish*.
*You can skip this step and leave it in the pot if you want to dirty fewer dishes. Spreading it in a larger pan simply allows the breadcrumbs to cover more surface area of the casserole, meaning more crunchy topping for everyone!