Prepare rice according to package directions.

In a large pot, saute onion in butter a few minutes. Add chicken broth and evaporated milk and bring to a boil.

In a small cup, dissolve cornstarch with 4 tablespoons water and whisk into hot broth. Stir constantly for l minute; reduce heat to low and stir in chicken, broccoli, mushrooms and rice. Season generously with salt & pepper. Continue cooking on low until host and ready to serve.