Heat your oven to 400F.
Bring 1/2 cup of water to a boil. Add the porcini mushrooms and let sit for 20 minutes, until plump. Remove the mushrooms and finely chop them.
Melt 1 Tablespoon of butter and oil in a saucepan over medium heat. Add the porcini and cremini mushrooms and the garlic. Cook for 6-8 minutes, until softened.
Sprinkle the flour over the mushrooms, stirring to coat them.
Slowly whisk in the broth and heavy cream. Simmer for 10 minutes, until thick enough to coat the back of a spoon.
Add the green beans and simmer 4-5 minutes more.
Meanwhile, heat the remaining tablespoon of butter in a skillet over medium-high heat. Add the onion. Cover and cook 3-4 minutes, until the onion begins to soften. Remove the cover and continue to cook, stirring frequently, for 5-6 minutes or until a rich golden brown. If the onions start to stick to the pan, stir in a tablespoon or two of water.
Stir half of the onions into the green beans mixture, then transfer to a medium (roughly 8x6) baking dish. Scatter the remaining onions over the top, then cover with an even layer of panko.
Bake for 15-20 minutes, or until golden brown and bubbling.
Let stand 10 minutes before serving.