Heat broiler to low. Coat a baking sheet with cooking spray. Season salmon with 1/2 teaspoon salt and 1/8 teaspoon pepper.
Broil until cooked through, 5 minutes per side. Let cool; cut salmon into bite-size pieces. Cook pasta as directed on package; add peas 3 minutes before end of cooking time; drain.
Whisk 1/4 cup milk and flour in a small saucepan until smooth; whisk in remaining 1 1/4 cups milk, remaining 1/4 teaspoon salt and remaining 1/8 teaspoon pepper. Cook over medium heat, whisking, until as thick as heavy cream, 10 minutes.
Remove from heat; add Neufchaåtel and 1 tbsp dill; whisk until cheese melts. Toss pasta and peas with sauce and salmon; drizzle with juice. Garnish with zest and remaining 1 tablespoon dill.