Position a rack in the center of the oven and heat the oven to 350°F.
Put 4-1/2 cups water and 1/2 tsp. salt in a 4-quart saucepan, cover, and bring to a boil. Whisk the grits into the pan in a slow stream. Reduce the heat to medium low, cover, and simmer, whisking occasionally, until thickened, 12 to 15 minutes.
In a large bowl, toss 1-1/4 cups of the cheese, the flour, thyme, and several grinds of pepper. Chop the garlic, sprinkle with a generous pinch of salt, and mash it into a paste with the side of a chef’s knife. Whisk the mashed garlic, the cheese mixture, and the bacon into the grits until blended and the cheese is melted. Season to taste with salt and pepper.
Scrape the grits into the large bowl. In a medium bowl beat the egg whites and a pinch of salt with a hand mixer until they just hold stiff peaks. In a small bowl, whisk the yolks and cream; whisk this mixture into the grits. With a large spatula, gently stir one-third of the whites into the grits to lighten them and then fold in the remaining whites. Scrape the grits into an 8x8x2-inch glass or ceramic baking dish.
Sprinkle the remaining 1/4 cup cheese evenly over the grits. Bake until puffed, browned, and bubbling, 50 minutes to 1 hour.