In a medium saucepan, bring 4 cups of water to a boil. Gradually stir in the grits and reduce the heat to low. Cook according to the package instructions until thickened, about 5-10 minutes, stirring occasionally.
Once the grits are cooked, remove them from heat and stir in the shredded cheddar cheese until melted and creamy. Season with salt and pepper to taste and cover to set aside.
In a large skillet, cook the bacon over medium heat until crispy. Remove the bacon and crumble it, leaving the rendered fat in the skillet.
In the same skillet, sauté the chopped onion and bell pepper in the bacon fat for about 5 minutes until softened. Add the minced garlic and cook for an additional minute.
Add the shrimp to the skillet and sprinkle with Cajun seasoning. Cook for 3-4 minutes or until the shrimp are pink and cooked through. Season with salt and pepper to taste.
To serve, spoon the cheesy grits onto plates, top with the sautéed shrimp mixture, and sprinkle with crumbled bacon, sliced green onions, and parsley if desired.