This wonderfully hearty stuffing can double as a vegetarian main dish for Thanksgiving. Just substitute mushroom broth or water for the chicken stock. Slideshow: More Vegetarian Recipes
Preheat the oven to 400°. Butter a 4-quart baking dish.
In a medium saucepan of salted boiling water, cook the farro until al dente, about 20 minutes. Drain well; transfer to a very large bowl.
Meanwhile, spread the bread on a large baking sheet and toast until golden and crisp, about 15 minutes. Transfer to the bowl.
In a large nonstick skillet, melt 2 tablespoons of the butter in 2 tablespoons of the olive oil. Add the leeks and garlic and cook over moderate heat, stirring occasionally, until softened, about 8 minutes. Stir in the mustard greens in batches and cook until wilted. Season with salt and pepper. Add the vegetables to the bowl.
In the same skillet, melt 1 tablespoon of the butter in 1 tablespoon of the olive oil. Add half of the mushrooms and season with salt and pepper. Cook over moderately high heat, stirring, until golden, about 3 minutes. Transfer to the bowl. Repeat with the remaining 1 tablespoon of butter, 1 tablespoon of oil and mushrooms.
Add the stock, lemon juice, eggs, scallions and 1 cup of the cheese to the bowl and mix well. Transfer the stuffing to the prepared baking dish and cover with foil. Bake for 40 minutes. Scatter the remaining cheese on top and bake uncovered for 25 to 30 minutes longer, until golden.
The assembled stuffing can be covered and refrigerated overnight.