Heat a pot of salted water to boiling then cook the pasta until just tender, drain and set aside.

While the water is heating for the pasta and for the pasta to cook, go ahead and heat a large non stick pan then add the bell peppers, onion, and ground beef, when the beef is good and browned, drain, then mix in the taco seasoning and add the drained tuna and mushrooms. Cook until the mushrooms are wilted and softened. Remove from heat and mix into the drained pasta, set aside.

In a medium saucepan set over medium heat, warm the milk until it is simmering then set it aside.

In a large heavy-bottomed stock pot, melt the butter over medium heat. Once the butter has melted, whisk in the flour and cook the mixture, whisking constantly, until it turns light brown in color and has a slightly toasted aroma, about 3 minutes. Whisk in the dry mustard.

Remove the pot from the heat then slowly whisk the warm milk into the flour mixture. Continue whisking constantly to avoid any lumps. (The mixture will become very thick and then thin out gradually until it is smooth.)

Return the pot to the stove over medium-high heat and cook the mixture while whisking constantly until it has thickened enough to coat the back of a spoon, about 2 minutes. Season the mixture with salt and pepper to taste and reduce the heat to medium. Whisk in the Cheddar and Parmesan cheeses until they are melted and the cheese sauce is smooth.

Add the pasta mixture to the cheese sauce and stir to mix together and heat to warm everything through, serve.