Bring the stock to a boil in a medium sized pot, then turn the heat down to low and slowly pour in the cornmeal while stirring briskly with a wooden spoon. Stirring while pouring is crucial to keep the grits creamy and smooth no lumps! Once the grits become smooth and thick, place a lid on the pot with the spoon still in it so that steam can escape.
Let the grits cook while you prepare the rest of the meal, checking in occasionally to give them a stir. The total cooking time should be about 25 to 30 minutes.
Meanwhile, prepare the shrimp and vegetables. Add the butter or oil to a large pan on medium heat and let it get hot. Add the onion, pepper, and celery. Sauté until the onion is just translucent, about 2 minutes. Add the garlic, as well as the green chile if you like things spicy. Let everything cook for another minute. Add the tomato and cook for 3 or 4 minutes, until the tomato releases its juice and the vegetables resemble a thick and chunky sauce.
Add a dribble of water to keep everything loose and saucy, then toss in the shrimp. Cook, stirring occasionally, until the shrimp are pink all over. Add salt and pepper to taste.
Once the grits are ready, take them off the heat and add the cheese and most of the scallions, then stir. Ladle the grits into bowls and top with the shrimp, vegetables, and a few more scallions.