In a large pan heat the Olive Oil over medium high heat.
Add the Mushrooms and let cook until browned and reduced by about a 1/3.
Add the Onions and cook until soft and translucent.
Add the Garlic, Salt, Pepper, Italian seasoning and Dijon Mustard and let cook an additional 2 minutes.
Remove from heat and add the Vinegar, Bread Crumbs and Romano cheese, stir to combine.
Preheat oven to 350˚.
Spoon mixture into Zucchini halves. Top with an additional sprinkle of bread crumbs and cheese if desired.
Bake at 350˚ for 8 minutes. Then broil an additional 2-3 minutes until the tops are browned and the additional cheese, if added, is melted.
Serve hot.