Start by preheating your oven to 350°F to get it ready for baking the mac and cheese.
Cook the spaghetti noodles in boiling water until al dente, following the instructions on the package.
While the noodles are cooking, melt the butter in a large saucepan over medium heat.
Stir in the flour and cook for 1-2 minutes, making sure to stir constantly to prevent burning.
Gradually pour in the milk, adding salt and pepper, and whisk continuously to create a smooth sauce without lumps.
Allow the sauce to thicken on medium-high heat, which should take about 5-6 minutes.
Remove the sauce from heat and blend in the 1 1/2 cups of mixed shredded cheese until it's completely melted and the sauce is smooth.
Toss the cooked and drained spaghetti noodles into the cheese sauce until they are well coated.
Transfer the cheesy spaghetti into a baking dish, spreading it out evenly.
Sprinkle the top with the remaining mixed shredded cheese and Parmesan cheese.
Bake in the preheated oven for 22-25 minutes. For a golden top, broil for a few minutes at the end.
Let the mac and cheese cool slightly before serving. Store any leftovers in a sealed container in the refrigerator.