Prepare and start marinating the tomatoes early in the day. Cut the stem button out of each tomato and cut it in half lengthwise. Trim off a small slice on the round side of each tomato half and discard it. Then cut each half, lengthwise, into 2 slices, each about 1/2 inch thick. Pack the tomato slices into a bowl. In a saucepan, slowly stir the 2 cups of water into the mustard powder and tomato paste until the mixture is smooth. Add the 2 tablespoons of oil, the vinegar, soy sauce, honey, the 2 teaspoons of salt, the celery salt and cayenne.
Bring the mixture to simmer over medium heat then simmer it, uncovered, for 5 minutes. Pour the hot marinade over the tomatoes in the bowl and set a saucer with a weight upon it on them to keep them submerged, and allow them to marinate at least 6 hours. Just before serving time, drain the tomatoes in a large strainer set over a bowl.
Measure out the drained marinade liquid and add enough water to it if necessary to bring the total measurement to 3 cups. Dissolve the cornstarch in the liquid and bring it to simmer in a heavy saucepan over medium heat while stirring constantly, with a rubber spatula.
When the liquid simmers, reduce the heat slightly and simmer 2 minutes longer, still stirring constantly. The sauce will be very thick and is meant to be so. Keep the sauce warm, covered with a trimmed piece of paper towel and lid, over very low heat until serving. Heat 1/4 inch of oil in a heavy, 10-inch frying pan over medium high heat. There will be around 8 tomato slices per serving so work in batches of 8.
Meanwhile, lightly salt both sides of each slice, dredge it in a bowl of the flour and set it on a plate. (Do not dredge more than 8 slices ahead of time, they get soggy.) Fry the floured tomato slices for 2 to 2 1/2 minutes on each side until they are golden brown. Do not overcook them. They retain their heat for a long time and consequently continue to cook after they are removed from the pan.
Drain the fried slices for a moment on paper towels. Transfer them to a warm dinner plate and serve the fried green tomatoes, immediately, along with a small, warm bowl of the equally divided marinade sauce.