Preheat oven to 475°F. In a large mixing bowl, the onions, ¼ cup flour, panko and salt and toss to combine. Grease a baking sheet with a tablespoon of olive oil and evenly distribute the onion mixture. Bake the onions on the middle rack of the oven for about 30 minutes until golden brown, tossing the onions once or twice while they cook. Remove from the oven, set aside, and reduce heat to 375°F.
Bring a pot of salted water to a boil. Add the green beans and cook for about 5 minutes, until just crisp. Strain the cooking liquid and add cold water and a few pieces of ice to stop the cooking. When very cool, strain again and set aside.
In a medium frying pan, melt the butter and a tablespoon of olive oil over medium high heat. Add the mushrooms, season with salt and pepper, and cook until the water from the mushrooms has evaporated. Sprinkle 2 tablespoons of flour over the mushrooms and cook, stirring constantly until some of the raw flour flavor has cooked off. Add the half-and-half in slowly, mixing constantly to prevent the flour from getting lumpy. Bring the mixture to a simmer, taste and season with salt and pepper. Cook until the mixture thickens, about five minutes.
Remove from heat, stir in the ravioli and the green beans. Pour the mushroom, bean and ravioli mixture into the prepared casserole dish. Garnish with crispy onions and bake until heated through and golden brown, about 15 to 20 minutes. Serve hot.