On a large rimmed baking sheet, toss the shallots with 1/3 cup of the flour; shake off any excess flour. In a large, deep skillet, heat 1 inch of oil until shimmering. Add the shallots in 2 batches and fry over moderate heat until very crisp, about 3 minutes. Using a slotted spoon, transfer to paper towels, then sprinkle with salt.
In a large pot of boiling salted water, cook the beans until just tender, about 5 minutes. Drain and refresh under cold running water; drain and pat dry.
Melt the butter in a large, enameled cast-iron casserole. Add the onion and cook over low heat, stirring occasionally, until softened, about 5 minutes. Add the paprika, cayenne and a large pinch of pepper; cook for 1 minute, stirring. Add the mushrooms, cover and cook over moderate heat until softened, about 5 minutes. Uncover and cook, stirring, until browned, about 5 minutes longer. Stir in the remaining 3 tablespoons of flour and gradually stir in the stock until smooth.
Simmer the mushroom sauce over low heat, stirring, until thickened, about 5 minutes. Stir in the crème fraîche, lemon juice and beans. Cover and simmer, stirring occasionally, until the beans are heated through, about 5 minutes. Season with salt and pepper and transfer to a large glass or ceramic baking dish.
Preheat the oven to 400°. Cover the casserole with foil and bake until bubbling, about 20 minutes. Uncover, scatter the shallots on top and serve.
Cremini mushrooms are actually baby portobellos and can be replaced in this recipe with a variety of small mushrooms from white button to chanterelle.
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