How to Make It
Preheat oven to 200°F. Toss together shrimp and Cajun seasoning in a medium bowl. Cover; refrigerate until ready to use.
Place milk and 1/4 cup butter in a medium ovenproof saucepan. Bring to a vigorous simmer over medium, stirring occasionally. Gradually add grits, whisking constantly. Reduce heat to medium-low; cook, whisking constantly, until mixture thickens slightly and grits are evenly suspended in milk, about 6 minutes. Cover saucepan; transfer to preheated oven. Bake until grits are tender, 1 hour to 1 hour and 30 minutes. Remove from oven; stir in 2 teaspoons salt and remaining 1/4 cup butter. Cover to keep warm until ready to use.
Meanwhile, stir together vinegar, 3/4 cup water, sugar, and remaining 1 1/4 teaspoons salt in a saucepan; bring to a boil over high. Stir in mustard seeds. Reduce heat to medium-low; simmer, stirring occasionally, until seeds are tender, about 35 minutes. Add tomato and bell pepper; cook, stirring occasionally, until vegetables are tender and mixture has thickened, 14 to 16 minutes. Remove from heat.
Remove shrimp from refrigerator. Heat oil in a large cast-iron skillet over high until smoking. Add shrimp to skillet in a single layer; cook until opaque and lightly charred, about 1 minute and 30 seconds per side. To serve, divide grits among 4 bowls. Top evenly with shrimp and chowchow; sprinkle with sliced scallion.