Preheat oven to 425 degrees. Wash mini pumpkins and cut 2 slits in top to allow steam to escape. Place in 9 by 13 inch baking pan that has been sprayed with non-stick spray. Bake 30-40 minutes until tender. Remove from oven.
When pumpkins are cool enough to handle cut tops off and scoop out seeds. Place 1 teaspoon butter and some salt and pepper in each. Fill shells with cooked peas and onions. Return to warm oven in covered pan until ready to serve, but don't hold them very long or they lose color.