Preheat oven to 350.
Cook pasta according to package directions
Heat large skillet over medium heat. Add oil to pan; swirl to coat.
Add onion; cook 6 minutes or until tender, stirring occasionally.
Sprinkle with flour; cook 1 minute, stirring consistently.
Gradually stir in milk (¼ cup at a time till right consistency while stirring), cook 5 minutes, stirring constantly with a whisk until slightly thick.
Stir in cream cheese, mustard, salt, and pepper; cook 2 minutes, stirring constantly. (If it seems dry add in a bit of sour cream).
Remove pan from heat.
Stir in pasta, peas, ¼ cup parmigiano-reggiano cheese and tuna.
Spoon mixture into a shallow 2 quart baking dish coated with cooking spray. Top with remaining ¼ cup parmigiano-reggiano and breadcrumbs tossed in butter.
Bake at 350 for 30 mins.
