Bring 12 cups of water to a rolling boil in a large pot and add 1 tablespoon salt and the 14 ounces of spaghetti.
Cook the pasta according to package instructions minus 1 minute; in the final minute add the frozen peas to the boiling water.
Before draining, scoop out 1 cup of the hot pasta water and set aside, then drain the pasta and peas in a colander and shake off excess water.
While the pasta cooks, melt the butter in a large skillet over medium heat.
Sprinkle the flour over the melted butter and whisk until smooth; continue whisking for 1 minute to cook the flour.
Gradually whisk in the milk until the sauce is smooth and has thickened, then whisk in the garlic powder, onion powder, ground mustard, and a pinch of salt and pepper.
Remove the skillet from the heat and add the shredded Cheddar in handfuls, stirring after each addition until mostly melted.
Add the drained pasta and peas to the skillet and use tongs to toss until the sauce evenly coats the pasta.
If the sauce is too thick, gradually stir in reserved pasta water until you reach a creamy consistency; taste and adjust salt as needed.
Serve immediately topped with freshly grated Parmesan cheese.