Prepare dinner mix according to package directions.
In a small skillet, saute onions in 2 tablespoons butter, until tender.
Stir the chicken, peas, mayo, and sauteed onion into the dinner mix.
Transfer to a 1 1/2 quart baking dish.
Melt remaining butter, and toss with breadcrumbs.
Sprinkle over top, and bake uncovered at 350 degrees for 20-25 minutes or until bubbly.
