Preheat oven to 350°F (175°C) if baking.

Remove stems from mushrooms.

Rinse mushroom caps in a colander and drain.

Finely chop the mushroom stems.

In a skillet, heat margarine over medium heat.

Add diced onion and green pepper to the skillet and sauté until softened.

Add the chopped mushroom stems to the skillet and cook until tender.

In a bowl, combine the cooked vegetables, bread crumbs, chopped nuts (if using), salt, pepper, and garlic salt.

Mix well to combine all ingredients.

Spoon the mixture into the mushroom caps, filling each one generously.

Place the stuffed mushrooms on a baking sheet.

Bake for 10-15 minutes, or until heated through and lightly browned (optional).

Serve warm as an appetizer or side dish.