Preheat oven to 350F. Cook and stir onion and green pepper in 1 Tbsp. of the butter in large saucepan on medium heat until crisp-tender; remove from saucepan. Set aside.
Mix soup and milk in same saucepan; cook on low heat until heated through, stirring occasionally.
Add VELVEETA; cook until melted. stirring frequenty.
Add onion mixture, tuna, noodles and pimiento; mix lightly.
Spoon into greased 1-1/2-quart casserole dish. Melt remaining 3 Tbsp. butter; toss with bread cubes.