Bring 12 cups of water to a boil in a large pot. Add 1 tablespoon of salt and drop in the spaghetti. Cook according to package instructions, subtracting 1 minute for al dente.
In the last minute of cooking, add the frozen peas to the pot.
Before draining, reserve 1 cup of pasta water. Drain the spaghetti and peas in a colander.
In a large skillet over medium heat, melt the butter. Whisk in the flour until smooth, about 1 minute.
Gradually pour in the milk while whisking until thickened into a creamy sauce. Add garlic powder, onion powder, and ground mustard, seasoning with salt and pepper.
Remove from heat and stir in the Cheddar cheese until melted and smooth.
Add the drained spaghetti and peas to the skillet, tossing to coat in the cheese sauce. Gradually add reserved pasta water until desired consistency is reached.
Serve hot, topped with freshly grated Parmesan cheese.
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Notes
Store leftovers in an airtight container in the refrigerator for up to 3 days.