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1
For the topping: Pulse bread, butter, salt, and pepper in workbowl of food processor fitted with metal blade until mixture resembles coarse crumbs, about ten 1-second pulses.
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2
Transfer to large bowl and toss with onions; set aside.
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3
For the beans: Heat oven to 375 degrees. Bring 4 quarts water to boil in large pot. Add 2 tablespoons salt and beans. Cook until bright green and slightly crunchy, 4 to 5 minutes.
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4
Drain beans and plunge immediately into large bowl filled with ice water to stop cooking. Spread beans out onto paper towel-lined baking sheet to drain.
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5
Meanwhile, cover dried porcini with 1/2 cup hot tap water in small microwave-safe bowl; cover with plastic wrap, cut several steam vents with paring knife, and microwave on high power for 30 seconds. Let stand until mushrooms soften, about 5 minutes.
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6
Lift mushrooms from the liquid with fork and mince using chef's knife (you should have about 2 tablespoons).
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7
Pour liquid through paper towel-lined sieve and reserve.
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8
Melt butter in large nonstick skillet over medium-high heat until foaming subsides, about 1 minute. Add onion, garlic, button mushrooms, and cremini mushrooms and cook until mushrooms release their moisture, about 2 minutes.
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9
Add porcini mushrooms along with their strained soaking liquid, thyme, 1 teaspoon salt, and pepper and cook until all mushrooms are tender and liquid has reduced to 2 tablespoons, about 5 minutes.
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10
Add flour and cook for about 1 minute. Stir in chicken broth and reduce heat to medium.
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11
Stir in cream and simmer gently until sauce has consistency of dense soup, about 15 minutes.
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12
Arrange beans in 3-quart gratin dish. Pour mushroom mixture over beans and mix to coat beans evenly.
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13
Sprinkle with bread crumb mixture and bake until top is golden brown and sauce is bubbling around edges, about 15 minutes.
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14
Serve immediately.
