Spray a 3-quart baking dish with cooking spray and preheat the oven to 350 F.
Pour the chicken broth into a large saucepan and bring to a boil. Stir in the salt and grits, bring to a boil, reduce the heat, and simmer for 20 mins, stirring frequently.
Melt the butter in a large skillet and sauté the green onion, bell pepper, and garlic until tender, about 5 mins. Add the shrimp, oregano, red pepper flakes, Old Bay, and black pepper. Cook the shrimp for 2 minutes on each side. Remove from heat.
Into the cooked grits, add the sauteed veggies, shrimp, Monterey Jack cheese, ½ cup cheddar, and diced green tomatoes with green chilis. Stir to combine. Pour the mixture into the prepared baking dish, top with the remaining ½ cup of cheddar, and bake for 30 to 45 minutes or until bubbly and golden brown.