Preheat oven to 350 degrees Fahrenheit. In a 9×13-inch baking pan, combine the onion, tomatoes, mushrooms, oil, sugar, and salt. Roast, uncovered, until caramelized, 30-40 minutes.

Remove the pan from the oven and stir the roasted vegetables to combine. Add the gefilte fish loaf to the pan and bake, covered, for one hour and 30 minutes longer. Serve the gefilte fish warm, topped with the vegetables.