Pour the farro into a medium pan; add enough water to cover by 2 inches. Add the onion and bay leaf; bring to a rolling boil. Salt the water; reduce the heat to medium. Keep at a low boil until the farro is al dente, about 20 minutes. Drain; discard onion and bay leaf.
While the farro is cooking, preheat the oven to 400 degrees . Place the nuts in a single layer on a small rimmed baking sheet and bake until toasted, about 5 minutes. Let cool.
In a large skillet, heat the oil, three turns of the pan, over medium-high. Add the leeks and toss until they begin to soften, 1 to 2 minutes. Add the raisins and garlic; stir until the garlic is aromatic, about 1 minute. Remove from the heat; add the marsala and swirl to blend. Return to the heat; add the chard in large handfuls, stirring until wilted, about 2 minutes. Season with salt, pepper and nutmeg; remove from the heat.
Heat a large nonstick skillet over medium-high. Add half the prosciutto in a single layer. Cook until crispy, 1 to 2 minutes per side. Repeat with remaining prosciutto.
On a large platter, toss the farro, chard mixture and nuts; season. Break up the prosciutto and place on top.