In a medium saucepan, bring 12 cups water to a boil over medium-high heat. Slowly whisk in grits, butter, 1½ tablespoons kosher salt, and garlic powder. Reduce heat to medium-low; cook, stirring occasionally, until thickened, about 20 minutes. Stir in cheese until smooth.

In a large skillet, heat oil over medium-high heat. Add onion to pan; cook until tender, about 3 minutes. Add shrimp, garlic, lemon juice, and remaining 1 tablespoon salt; cook until shrimp are pink and firm, about 5 minutes. Stir in parsley and tomatoes. Serve shrimp over grits. Garnish with arugula and feta, if desired.