Pre-heat oven to 350° (180° celsius).Boil potatoes in salted water until tender but firm, remove to cool, in the same water add the broccoli florets and cook approximately 5 minutes. Drain well.While potatoes are cooling, make the white sauce.
In a medium pot over low heat melt butter, whisk in flour and salt, continue whisking until smooth. Slowly add the milk and continue whisking constantly until thickened and starts to boil. Remove from heat. Let cool slightly.In a 8x6 inch (20 x 15 centimeter) baking dish, spread a couple of spoonfuls of white sauce on the bottom, make 3 layers, starting with the sliced potatoes, spinkle of oregano, white sauce, broccoli florets (if too large cut into smaller pieces) and a sprinkle of parmesan cheese, continue for three layers. Bake for approximately 30 minutes (test for doneness after 20 minutes). Let cool 5 minutes before serving. Enjoy!