Heat olive oil in a large Dutch oven over medium-high heat. Add red onion, carrots, and celery and cook, stirring frequently, until the vegetables are tender and caramelized, about 8-10 minutes.

Add the tomato sauce and sugar and bring to a simmer. Reduce the heat to low and partially cover. Simmer for 30 minutes or up to 4 hours, stirring occasionally.

Meanwhile, in a large pot of boiling, salted water, cook the shells until just done, about 10 minutes. Drain and toss with the sauce. Put half of the pasta into the prepared baking dish and top ...

Diana Chistruga. Cook, uncovered, for 3 hours or more, stirring from time to time. While the sauce is cooking, you are likely to find that it begins to dry out and the fat separates from the meat.