In a small saucepan, bring the water and sea salt to a boil, then slowly whisk in the stone-ground grits.
Reduce the heat to low, cover the pan, and simmer for 15-20 minutes until the grits are creamy and tender, stirring occasionally to prevent sticking.
While the grits are simmering, heat the olive oil in a large skillet over medium heat.
Add the diced yellow onion, green bell pepper, and celery to the skillet and sauté until the vegetables are softened and fragrant.
Stir in the minced garlic and Cajun seasoning, cooking for another 60 seconds to release the aromatics.
Add the shrimp to the skillet in a single layer and cook for 2-3 minutes per side until they are pink, opaque, and slightly charred.
Stir in the lemon juice and black pepper, tossing the shrimp and vegetables together to coat.
Spoon the warm grits into a serving bowl and top with the Cajun shrimp mixture and fresh green onions.