Toss the squash and potatoes with 2 tablespoons of the oil, salt and pepper. Spread in a single layer on a large rimmed baking pan and roast, stirring once or twice, until tender and golden brown in spots, 15 to 20 minutes.
Meanwhile, heat the remaining tablespoon oil in a medium pot over medium-high heat. Add the onion, garlic, rosemary and a pinch of salt and cook, stirring, until softened, about 8 minutes. Add the tomatoes and roasted vegetables and simmer until slightly thickened and the flavors have blended, about 10 minutes. Transfer to a serving bowl.