Boil the pasta in heavily salted water in a 2-quart saucepan according to the package directions.
Drain the pasta and set it aside.
Heat the empty saucepan over low heat. Add the butter and cream cheese and melt for 1 minute, stirring frequently, or until the butter is melted and starting to foam. Note: it's ok if the butter and cream cheese look separated. They will melt together completely in the next step.
Add the cheddar cheese, milk, and salt to the pan. Increase the temperature to medium-low and let the mixture come to a simmer, stirring the mixture as it cooks. Simmer the sauce for 5-6 minutes, continuing to stir every 15-30 seconds or so, or until the sauce is thickened and creamy. Note: you want the sauce to simmer lightly, but not boil or burn. If it starts to boil turn the heat back down to low.
Remove the saucepan from the heat. Gently fold in the cooked pasta and flaked tuna until combined. Serve hot!
Notes
1. Making the recipe as directed makes it a one-pot recipe. You can also simmer up the cheese sauce in a separate pan while the pasta cooks to save time, however. 2. To make this recipe gluten-free, just use substitute gluten-free rice or chickpea pasta. 3. Leftovers last covered in the refrigerator for 3-4 days. They can also be frozen in a freezer-safe container for up to 2 months. 4. This recipe can easily be doubled or tripled. Just multiply your ingredients by the desired amount and follow the directions as listed, increasing the size of your pot to accommodate the extra volume, if needed.