Melt the butter in a skillet over medium low heat and add the chopped onion. Saute until the onion is transparent, about 10 minutes. Add the brown sugar, salt, pepper, and marjoram and stir well to combine. Add the bread cubes and toss to coat with the seasoned butter. Remove from the heat and reserve.
Place the tomatoes in a mixing bowl and break them up into chunks by crushing against the side of the bowl with the back of a heavy spoon. Add the tomato paste and blend until smooth. Reserve.
Preheat the oven to 350°F Generously butter a shallow 2 ½ quart baking dish. Spoon about half of the tomato mixture into the baking dish and top with half of the bread mixture. Repeat. Scatter the sliced onion over the bread cubes and dot with additional butter.
Bake for 35 to 40 minutes, or until tomatoes are thick and bubbling and onion is nicely browned. Serve warm. Serves 10-12.