Complex sentence: This method of preparing vegatables became very popular during the Great Depression in the United States and became cooked in a casserole form often with a light pie crust on top as in a traditional pot pie.
Split into simpler sentences: This method of preparing vegetables became very popular during the Great Depression in the United States. It was sometimes cooked in a casserole form, often with a light pie crust on top as in a traditional pot pie.