In a large skillet, heat the olive oil over medium-high heat. Add the onion and season with a bit of salt. Cook, stirring occasionally, until softened, about 2 minutes. Add the sausage and cook, breaking it up, until cooked through and starting to brown, 7-8 minutes. Add the 2 cloves of minced garlic, the oregano, crushed red pepper and the fennel seeds. Cook, stirring, until fragrant, about 1 minute. Stir in the vinegar.
For slow cooker: Spoon mixture into a 6-quart slow cooker. Add the farro, crushed tomatoes and water. Season with salt and pepper, the cover and cook on high until the farro is tender, about 2 - 2 1/2 hours. Stir in the green onions.
Alternately, you can simmer this one covered in a Dutch oven on the stove-top or in a 350F oven, checking every 30 minutes or so to stir and check farro for doneness.
For the topping: heat the olive oil over medium-high heat. Add the panko and cook, stirring, until golden brown, about 3 minutes. Add the parsley, lemon zest, fresh oregano and garlic. Cook, stirring, until the garlic is tender, 1 minute. Season with salt and freshly ground pepper. Transfer to a plate to cool and set aside.
When farro mixture is cooked, spoon into an oven-safe baking dish (or smaller, individual baking dishes). Sprinkle with grated Parmesan and top with mozzarella slices. Place under your oven broiler until the cheese is melted and starting to brown, about 5 minutes. Let stand a few minutes, then sprinkle with panko to serve.