Pre-heat oven to 350° (180° celsius). Dot the bottom of a 9x12 (23x30cm) baking dish with approximately 1 tablespoon of butter, set aside.
Cook the Cannelloni tubes in boiling salted water until very al dente, drain and towel dry before stuffing.
MUSHROOM FILLING
In a medium frying pan over medium heat add the olive oil, mushrooms, oregano, parsley, salt and water, cook until tender and liquid has evaporated, add chopped ham and continue cooking for approximately 2 minutes. Remove from heat, let cool then fold in the cubed mozzarella and ¼ cup parmesan cheese.
WHITE SAUCE
In a melted pot add the butter and heat on medium until melted, add the flour and salt and whisk to combine, slowly add the milk and continue whisking until thickened. Remove from heat. Place ½ a ladle of white sauce on the bottom of the baking dish.
Stuff the cannelloni tubes with 2-3 dessert spoons of mushroom/cheese mixture, place the cannelloni in a single layer in the prepared baking dish continue until finished, then spoon the remaining white sauce over the cannelloni and sprinkle with the remaining ¼ cup freshly grated parmesan. Bake for approximately 20-25 minutes. Let sit 5 minutes before serving. Enjoy!