1. Sauté onion in a large skillet until golden.
2. Add tomato paste, brown sugar, and spices and mix until combined. Cool for 5–10 minutes.
3. Stuff pasta shells with vegetable filling and place seam-side down in two 9×13-inch (20×30-centimeter) pans.
4. Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius).
5. Add the rest of the vegetables and sauté an additional 5–7 minutes over medium heat so that the liquid evaporates while cooking. Be sure to stir every so often, sautéing until slightly soft but not mushy. Keep over medium heat so that the liquid evaporates while cooking.