Saute ground beef with diced onions and sliced mushrooms. Do not drain meat, onions or mushrooms. Add the garlic powder to the undiluted cream of mushroom soup and stir well. Add soup mixture to the meat, mushrooms, and onions in pan, and add the chopped red peppers, stirring until thoroughly mixed--juices will hold it together.
Preheat oven to 375 degrees Fahrenheit.
Into individual crocks or bowls, spread the frozen green beans on bottom of each crock (green peas may be substituted). Fill crocks with meat mixture. Top each crock with 1 cup of mashed potatoes, in a big dollop or piped through a pastry bag to make them more elegant. Bake until potatoes are crisp and slightly browned, about 20 to 25 minutes.
The shepherd's pies may be stored, unbaked and covered, for up to 3 days, in the refrigerator.