1. Begin by preheating the oven to 375 degrees.
2. On the stove top with a large sauté pan the 1 TBLS of olive oil over medium heat. Add in the ½ sliced onion and a pinch of salt and let this cook for 3-4 minutes. Then add in the sliced mini sweet peppers, green beans, and mushrooms. Add a pinch of salt and pepper and let this cook for 7-8 minutes on medium heat. Once the vegetables have cooked remove the sauté pan from the stove top.
3. To prepare the sauce, heat the olive oil or butter in a sauce pan over medium heat. Once the olive oil or butter is hot adding in the flour and whisk together. Then carefully pour in the milk, salt, and pepper. Stir everything together and bring it to a light boil for the sauce to thicken. Once the sauce has thickened remove the sauce pan from the stove top and carefully pour this over the cooked vegetables and toss together.
4. Add the sauced vegetables into a casserole or oven proof baking dish.
5. For the crispy topping use a medium sauté pan. Heat the olive oil or butter over medium heat, once hot add the onions with a pinch or 2 of salt to allow them to sweat. Let the onions cook for 4-5 minutes. Then add the panko bread crumbs to the sauté pan and stir everything together. Do not worry the panko will not perfectly adhere to the onions. Once the panko has lightly cooked for 2-3 minutes turn the heat off, toss in the grated Parmesan cheese, stir together and pour the crispy topping over the casserole. Bake the casserole for 20-25 minutes and then serve.