As I mentioned above, you can use any firm, white fish here. Grouper or striped bass are ideal substitutes, but snapper, catfish, walleye, redfish, seabass or Pacific rockfish or lingcod are great, too. Get the best grits you can afford.
Boil the water and stock and add salt to taste. Start stirring it and pour the grits in slowly; this prevents lumps. Drop the heat to a simmer and cook, stirring frequently, for about 20 to 30 minutes.
Meanwhile, salt the fish fillets and set aside on the counter. Fry the bacon in a large pan to render the fat. Remove the bacon, chop and set aside.
Pat the fish dry with paper towels and place in the hot bacon fat, the flattest side down. Keep the heat at medium-high. Use a spoon to ladle bacon fat over the top of the fillets until they turn opaque. Keep doing this for about 1 minute. Do not flip your fish unless the pieces are more than 1 inch thick. When you have a nice sear (see picture above), remove the fish from the pan and set, seared side up, on a cutting board.
If you have less than about 2 tablespoons of bacon fat in the pan, add some more. Add the mushrooms and sear them well. They'll sear, then give up their water. Let this happen without moving the mushrooms. This should all take about 3 minutes or so.
Right before serving, stir the butter, cream and cheese into the grits until the butter and cheese melts in.
When the mushroom water subsides, add the remaining ingredients and stir-fry them about 90 seconds. To serve, give everyone some grits, then some fish, then pour the mushroom-tomato mixture over.