Cook first 3 ingredients in a large nonstick skillet coated with cooking spray, covered, over low heat 45 minutes or until onions are very tender, stirring occasionally. Remove mixture from heat, and set aside.
Reserve 2 tablespoons soymilk, and set aside. Bring remaining soymilk, unpeeled onion, and next 4 ingredients to a gentle boil in a medium saucepan; reduce heat, and simmer 15 minutes.
Melt butter in a nonstick skillet over medium heat. Stir in flour, and cook, stirring constantly, 2 to 3 minutes or until flour is lightly browned.
Pour soymilk mixture through a wire mesh strainer into a bowl, discarding solids. Whisk soymilk mixture into butter mixture; bring to a boil. Reduce heat, and simmer 6 minutes or until thickened.
Stir together cornstarch and reserved 2 tablespoons soymilk until smooth. Whisk cornstarch mixture into soymilk mixture, whisking until thickened. Remove from heat. Stir in diced onion mixture, salt, and pepper.
Note: Reduce sodium by omitting salt when preparing this recipe. Packaged polenta contains salt.