HEAT oil in large nonstick skillet and cook green peppers and onion, stirring occasionally, until golden, about 6 minutes. Remove pepper mixture from skillet and set aside.
PREPARE Knorr® Pasta Sides™ - Stroganoff in same skillet covered, according to package directions. Top with roast beef, then cheese. Arrange pepper mixture around edge of skillet. Cover and simmer until cheese is melted, about 2 minutes.