Preheat your oven to 400°F / 200°C.
Grease a 2-quart gratin dish or a 9x13-inch baking pan.
Place a large, deep skillet over medium heat.
Add 3 tablespoons (45 ml / 1.5 fl oz) of the oil or butter.
Once hot, add the cubed celery root and cook, stirring occasionally, until it begins to brown, about 8 minutes.
Add the chopped onion to the skillet, sprinkle with salt and pepper, and continue to cook until both the celery root and onion are soft and golden, about 3-5 more minutes.
Remove the skillet from the heat.
Stir in the chopped garlic, drained white beans, paprika (if using), and marjoram. If the mixture looks too dry, add a splash of the reserved bean liquid to create a thick, stew-like consistency.
Taste and adjust seasoning with more salt and pepper if needed.
Spread the bean and vegetable mixture evenly into your prepared baking dish.
Sprinkle the top with Parmesan cheese (if using) and then the bread crumbs.
Drizzle with the remaining 1 tablespoon (15 ml / 0.5 fl oz) of oil or butter.
Bake for 45 to 55 minutes, or until the top is golden brown and the edges are bubbling.
Let it rest for a few minutes before serving. It can be served hot or at room temperature.
Don't discard the liquid from your cooked or canned beans! It's full of flavor and starch, perfect for creating a creamy sauce for your gratin without adding dairy.
