1/4 cup extra virgin olive oil or 4 tablespoons (1/2 stick) butter, plus more for greasing the baking dish2 cups celery root, peeled and cut into 1-inch cubes 1 onion, chopped Salt and freshly ground black pepper 2 cloves garlic, chopped 3 cups cooked or canned white beans, drained but still moist, liquid reserved 1 teaspoon sweet or Spanish smoked paprika (optional) 1 teaspoon dried oregano 1/2 cup freshly grated Parmesan cheese, plus more for garnish 1/2 cup bread crumbs
Grease a 2-quart gratin dish or a 9 x 13-inch baking pan with butter or oil. Preheat the oven to 400°F. Put 3 tablespoons of the oil or butter in a deep skillet over medium heat. When hot, add the celery root and cook until it starts to brown, about 8 minutes. Add the onion and sprinkle with salt and pepper; cook until the celery root and onion are soft and golden, another 3 minutes or so.
Turn off the heat and stir in the garlic, beans, paprika, and oregano; add the reserved bean liquid if the mixture is too dry (it should be like a thick stew). Taste and adjust the seasoning.
Spread the bean and vegetable mixture in the prepared pan. Top with Parmesan and bread crumbs and drizzle with the remaining tablespoon of oil or butter. Bake until the edges and top are browned and bubbling, 45 to 55 minutes, depending on how deep your baking dish is. Serve immediately or let rest for up to an hour and serve at room temperature.
From How To Cook Everything Vegetarian by Mark Bittman