Bring water or water and milk mix with salt to a boil in a medium pot.
Slowly whisk in the grits, reduce heat to low, and simmer, stirring frequently, until thick and creamy, about 15 to 20 minutes.
Stir in butter and cheese if using, then cover and keep warm.
Heat butter or oil in a skillet over medium-high heat.
Add chopped onion and cook until softened.
Stir in minced garlic and cook for about 1 minute until fragrant.
Add shrimp, season with salt, pepper, and Cajun or Creole seasoning.
Cook until shrimp turn pink and opaque, about 2 to 4 minutes.
Finish with a splash of lemon juice or hot sauce if desired.
Spoon creamy grits into bowls, top with cooked shrimp, and garnish with parsley or green onions.