In a medium pot, bring chicken broth and milk to a simmer.
Slowly whisk in the grits to avoid lumps. Reduce heat to low.
Stir occasionally until thickened, about 20–25 minutes (or follow package directions if using quick-cooking grits).
Stir in butter and shredded cheddar cheese until smooth and creamy. Season with salt and pepper to taste. Keep warm.
Pat shrimp dry and toss with 1 tablespoon Cajun seasoning.
Heat olive oil in a large skillet over medium-high heat.
Sear shrimp for about 2 minutes per side until just cooked through. Remove and set aside.
In the same skillet, add Andouille sausage and cook until browned. Remove and set aside with shrimp.
In the same skillet, add onions and bell peppers. Cook until softened, about 5 minutes.
Stir in garlic and cook for 1 minute.
Sprinkle in smoked paprika, remaining Cajun seasoning, and crushed red pepper flakes. Stir to coat.
Pour in chicken broth and heavy cream. Bring to a gentle simmer and let thicken, about 5–7 minutes.
Return sausage and shrimp to the skillet. Toss to coat in the sauce. Taste and adjust seasoning with salt and pepper.
Spoon creamy cheddar grits into bowls.
Top with Cajun shrimp, sausage, and peppers in sauce.
Garnish with fresh parsley and green onions.