For the casserole: Preheat the oven to 375 degrees F.
Cook the pasta in a large pot of boiling water until less than al dente; set aside.
Put the butter and olive oil in a large pot over medium heat. When the butter has melted and the oil is hot, add the celery, oregano, garlic, carrots and onion and cook until the onions are translucent, about 5 minutes. Sprinkle over the flour, stir and cook for an additional minute. While whisking constantly, slowly add the stock. Bring to a simmer and allow the sauce to thicken, about 2 minutes. Season with salt and pepper. Add the meatballs, kale, Parmesan and cooked pasta, then stir to combine. Transfer to 9-by-13-inch casserole dish.
For the topping: Mix together the breadcrumbs, Parmesan, butter, parsley and oregano in a bowl until well combined. Sprinkle the breadcrumb topping over the casserole.
Bake until golden and bubbly, about 20 minutes. Allow the casserole to sit for 10 minutes before serving.