Add the farro and water to a medium saucepan. Bring to a boil then cover, reducing the heat to medium-low. Let simmer until the farro is tender and a little chewy, 30 to 35 minutes. Drain any excess water and add to a large bowl.
While the farro is cooking, start on the topping. Set a medium skillet over medium low heat. Add in the olive oil. When hot, add in the sliced onions and sprinkle with the salt. Cook for about 15 to 20 minutes, stirring occasionally, until very tender and the edges are starting to turn golden brown. Transfer the onions to a small bowl or plate and set aside.
In a small bowl, whisk together the tomato sauce, peach preserves, molasses, vinegar and brown sugar. Set the same skillet you used for the onions back over medium heat. Add in the olive oil. When hot, add in the onion and pepper. Cook until tender, about 5 minutes. Add the garlic and chipotle. Cook until fragrant, about 30 seconds. Pour the tomato sauce mixture into the skillet. Lower the heat slightly and let cook until thickened, 15-20 minutes. Add the sauce to the bowl with the farro and mix to combine.
Spoon some of the farro on the bottom of a bun. Top with onions and cover with the other half of the bun, then serve.
This recipe calls for pearled farro. If you use semi-pearled or whole farro, the cooking time will be longer. Depending on your taste, you may want to cut back on the chipotle. As written, it has a little bit of kick.