Butter a 2-quart baking dish and heat the oven to 375 F.
Cut the cabbage into large chunks and cut out the core pieces. Put the cabbage in a large saucepan and cover with water. Add 1 teaspoon of salt to the water and bring the cabbage to a boil over high heat. Reduce the heat to medium-low, cover the pan, and cook the cabbage for 8 to 10 minutes, until tender. Drain in a colander and set aside to cool.
In the same saucepan over medium heat, melt 2 tablespoons of butter. When the butter sizzles, add the flour, 1/2 teaspoon of salt, 1/4 teaspoon of pepper, and a dash of nutmeg; stir and cook for 2 minutes.
Gradually whisk the milk into the butter and flour mixture. Continue cooking, whisking constantly, until thickened.
Taste the sauce and adjust the seasonings.
When the cabbage is cool enough to handle, chop it coarsely and transfer it to the buttered baking dish. You should have approximately 5 to 6 cups of cabbage.
Pour the sauce mixture evenly over the cabbage and stir lightly.
Combine the panko crumbs with 2 tablespoons of melted butter and sprinkle over the cabbage.
Bake the scalloped cabbage for 25 minutes, until the topping is browned and the filling is bubbling around the edges.