In a large pot, bring 1 inch of water to a boil. Insert a vegetable steamer basket and place the frozen cauliflower, bell peppers, corn, and butternut squash in the basket. Cover and cook for 8 to 10 minutes until the vegetables are fork-tender.
Transfer the cooked vegetables to a blender. Add ½ cup of the milk. Blend until smooth and no visible pieces of vegetables remain, adding splashes of water or milk as needed to promote blending. The sauce should be thin but still coat the back of a spoon (due to the fiber from the vegetables). See notes for using an immersion blender instead if desired.
Drain the water you used for steaming the vegetables from the pot. Pour the blended vegetable mixture into the pot. Add the rest of the milk, butter, onion powder, garlic powder, and salt.
Bring the mixture to a boil, then quickly reduce the heat to maintain a gentle simmer (you don't want to scorch the milk).
Add the pasta to the pot. Cook, stirring frequently to prevent scorching, for anywhere from 8 to 20 minutes until the pasta is tender but still has a little "bite" (al dente). Cooking time will vary based on the type of pasta you use, so check for doneness at 7 or 8 minutes and go from there. I have found that elbow pasta cooks very quickly in the sauce, while bowtie pasta takes a while longer. Avoid overcooking, because pasta that is too soft will not hold the sauce as well and may turn soggy. Add some splashes of milk or water throughout the cooking process if you think the liquid is getting too low in the pot. There should always be enough liquid to maintain a gentle simmer.
When the noodles are cooked to your liking, remove the pot from heat. Stir in the cheddar cheese. The sauce should be thick and creamy. If you want to thin the sauce, add some splashes of water or milk and stir until it's your desired consistency.
Serve with an optional garnish of parsley, and enjoy!
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