Bring 4 cups of water and salt to a boil in a medium pot over high heat. Stirring constantly, add grits in a slow, steady stream. Lower heat to maintain a rapid simmer and cook, stirring, until water is absorbed and grits are smooth, about 5 minutes. Remove from heat and stir in butter and 1 1/2 cups cheddar. Season to taste with salt and pepper.
Preheat broiler. Divide grits among six small, shallow, ovenproof bowls. Sprinkle evenly with bacon and top with a tomato slice. Sprinkle with remaining 1/2 cup cheese. Transfer bowls to a rimmed baking sheet and broil until cheese melts and is bubbling, about 1 minute. Serve immediately.