Boil noodles for about 4-5 minutes and drain. (You want them to still have a fair amount of chew to them. Baking will soften them further and the noodles won't go mushy) Add butter, soups, cheese packet, and sour cream to noodles and stir to combine. Add only enough to thin it to a 'thick soup'' consistency.
Add mustard and 1/4 to 1/2 of the shredded cheese and the tuna (drained). Mix in. Add salt and pepper to taste.
Pour mixture into a 13x9 inch pan and top with remaining cheese and crushed chips.
Bake at 350 till casserole is bubbly, melted and crispy. Cool slightly before serving.