Put the farro in a fine-mesh sieve, rinse under cold running water, and shake dry thoroughly.
In a large sauté pan, melt 1 tbsp of the butter with the oil over medium- high heat. Add the onion and a generous pinch of salt and sauté until tender, about 5 minutes. Add the farro and stir for about 1 minute to coat with the fat. Pour in 1½ cups (12 fl oz/375 ml) of the broth and let come to a boil. Reduce the heat to medium-low heat, cover, and simmer for 15 minutes.
Uncover and stir in the squash and the remaining 1 cup (8 fl oz/ 250 ml) broth. Raise the heat to high and bring to a boil, stirring, then reduce the heat to medium-low, cover, and simmer until the squash is tender and the grains are tender but still chewy, 15–20 minutes longer. If any broth remains in the pan, cook, uncovered, until it is absorbed.
While the farro is cooking, melt the remaining 1 tbsp butter in a small frying pan over medium heat. Add the pecans, rosemary, and a pinch of salt and sauté until the nuts smell toasty, about 1½ minutes. Set aside.
When the farro is ready, transfer to a warmed serving dish, top with the nuts, and serve right away.