Heat the oven to 375°F and grease a 9 by 13 inch baking dish. Toss the panko with the 2 tablespoons of melted topping butter. Boil 4 quarts of water with 2 tablespoons of salt, then cook the beans for 4 minutes, until bright green and crisp tender. Move them to the ice bath for 3 minutes, drain, and pat dry with a towel.

Heat 1 tablespoon of butter and the oil in a skillet over medium high heat for 2 minutes. Add the mushrooms in a single layer and cook 6 to 8 minutes without crowding, stirring only twice, until deep golden and the liquid has cooked off. Do not salt them yet.

Add the remaining 3 tablespoons of sauce butter, the shallot, garlic, thyme, and tomato paste, and cook for 1 minute, until fragrant. Sprinkle in the flour and stir for 1 to 2 minutes, until it smells nutty and no dry flour remains.

Whisk in the broth, then the milk and cream, a little at a time, until smooth. Simmer for 5 minutes, until the sauce coats the back of a spoon, then stir in the nutmeg, salt, pepper, and lemon juice.

Fold the dry blanched beans into the sauce until every piece is coated, then pour the mixture into the baking dish. The sauce should look thick and creamy, not runny.

Bake uncovered at 375°F for 20 minutes, until the edges bubble. Scatter 1 cup of the fried onions and the buttered panko over the top, and bake 10 more minutes, until the topping is deep golden.

Let the casserole rest for 5 minutes so the sauce settles. Sprinkle with the remaining fried onions and parsley, and serve hot.