In large pot of lightly-salted water, boil macaroni according to package instructions. In a medium saucepan, melt butter over medium heat; stir in flour, salt and pepper, and slowly add milk. Cook and stir until bubbly. Stir in cheese until melted.
Add frozen peas to pasta water during the last few minutes of cooking. Drain macaroni and peas; add to cheese sauce. Shred tuna with fork and add to macaroni and peas, stir to combine.