In a large saucepan, heat 3 tablespoons of the oil. Add the onion and cook over moderately high heat until softened, about 2 minutes. Add the garlic and cook, stirring, until fragrant, about 1 minute. Add the farro and stir until coated with oil, about 1 minute. Add the white wine and simmer until almost evaporated, 2 minutes. Lower the heat to moderate. Add 1 cup of the water and cook, stirring, until absorbed. Repeat with the remaining water, adding 1 cup at a time and stirring constantly until the farro is just tender and suspended in a creamy sauce, 30 minutes total.