Melt margarine in large saucepan over medium heat and cook onion until tender, about 5 minutes. Stir in flour with wire whisk and cook over low heat, stirring constantly, 2 minutes. Whisk in Knorr® Homestyle Stock Chicken and water until blended. Bring to a boil over high heat. Reduce heat to low and simmer, stirring occasionally with wire whisk, until thickened, about 8 minutes. Stir in Hellmann's® 1/2 the Fat mayonnaise-type dressing and cheese and cook, stirring occasionally, 3 minutes.
Combine bowtie pasta with sauce mixture in large bowl. Stir in tuna and peas; turn into 2-quart baking dish, then sprinkle with bread crumbs.
Bake uncovered until golden and bubbling, about 20 minutes.