Cook linguine in water with the tablespoon salt, adding peas for the last 2 minutes. Reserve 1 cup cooking water and drain when the pasta is just tender.
Meanwhile, whisk milk and cornstarch in a cold skillet until smooth. Add cream cheese, garlic powder, sauce salt and pepper.
Warm over medium-low heat for 4 to 5 minutes, whisking, until the cream cheese melts and the sauce reaches a gentle simmer and lightly thickens. Lower heat if it starts bubbling hard.
Remove from heat and stir in Parmesan gradually. Add pasta and peas and toss, using reserved cooking water a tablespoon at a time only if needed to loosen the sauce.
Serve promptly while the light coating spreads easily along the noodles.