In a large skillet, heat 1 tablespoon of the olive oil. Add the onion, season with salt and cook over moderately high heat, stirring occasionally, until softened, 3 minutes. Add the sausage and cook, breaking it up, until cooked through, about 7 minutes. Add two-thirds of the garlic, the dried oregano, crushed red pepper and fennel seeds. Cook until fragrant, about1 minute.
Stir in the vinegar, then scrape into a 6-quart slow cooker. Add the farro, crushed tomatoes and 2 1/2 cups of water. Season with salt and pepper. Cover and cook on high until the farro is tender, 2 1/2 – 3 hours. Stir in the scallions.
Meanwhile, in a medium skillet, heat the remaining 2 tablespoons of olive oil. Add the panko and cook over moderately high heat, stirring, until golden brown, about 3 minutes. Add the parsley, lemon zest, fresh oregano and the remaining garlic. Cook, stirring, until the garlic is tender, 1 minute. Season with salt. Transfer to a plate.
Preheat the broiler and position a rack 6 inches from the heat. Scrape the farro mixture into a 3-quart oval baking dish or 4 small baking dishes. Sprinkle with the grated Parmesan and top with the mozzarella. Broil until the cheese is melted and starting to brown, about 5 minutes; rotate the baking dish halfway through broiling. Transfer to a rack and let stand for 10 minutes. Sprinkle with the panko and serve.
One Step To Make Ahead: The cooled panko can be kept in an airtight container at room temperature overnight.