You can vary this recipe with different cheeses. The times given in the recipe are based on a 1100 watt microwave oven. If you are using a microwave with a different wattage, you can adjust the time with the help of a time conversion chart found here: https://www.microwavecookingforone.com/Charts/
Cook pasta in a large pot of boiling water. Add 1 teaspoon / 5 ml salt to the cooking water.
While the pasta is cooking, prepare the cheese sauce. In a ceramic or glass bowl, stir together cheese, cornstarch, salt, mustard, and pepper until thoroughly mixed. Add the milk and stir. (The cheese will sink to the bottom of the bowl.)
Heat the mixture in microwave oven on full power for 5 minutes. Stir. The sauce will be thin and the cheese will not be fully melted. Return to microwave and heat for 1 more minute. Stir. Return to microwave and complete cooking at 50% power for two minutes.
Remove sauce from microwave. The sauce will thicken slightly as it sits. Drain the pasta and pour sauce over to coat noodles. If using spinach, sprinkle chopped spinach over and stir into pasta. Serve hot.