Heat oil over medium heat in large saucepan. Add onion, and cook, stirring often, 8 to 9 minutes, or until softened. Add rice, 2 cups stock, mustard and 1 cup water. Cover, reduce heat to low and simmer 20 minutes, or until rice is tender.
Pour chowder base into food processor, purée until smooth, then return to pot (or use an immersion blender and purée in pot). Add remaining stock, vegetables, salt and pepper. Cover and cook 10 minutes, or until vegetables are tender. Stir in corn and basil, and cook until heated through. Serve.