Combine first 3 ingredients in a large saucepan over medium-high heat; bring to a simmer. Cover, reduce heat, and simmer 20 minutes.
Stir in oats; simmer, uncovered, 12 minutes, stirring often. Stir in grits; simmer 5 minutes, stirring often.
Stir in milk and syrup; simmer 1 minute or until porridge thickens, stirring constantly. Remove from heat. Serve with walnuts.