Green Bean Casserole

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. Transfer to large bowl and toss with onions; set aside.

For the beans: Heat oven to 375 degrees. Bring 4 quarts water to a boil in large pot. Add 2 tablespoons salt and beans. Cook until bright green and slightly crunchy, 4 to 5 minutes. 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.

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. Lift mushrooms from the liquid with fork and mince using chef's knife (you should have about 2 tablespoons). Pour liquid through paper towel-lined sieve and reserve.

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. 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. Add flour and cook for about 1 minute. Stir in stock and reduce heat to medium. Stir in cream and simmer gently until sauce has consistency of dense soup, about 15 minutes.

Arrange beans in 3-quart gratin dish. Pour mushroom mixture over beans and mix to coat beans evenly. Sprinkle with bread crumb mixture and bake until top is golden brown and sauce is bubbling around edges, about 15 minutes. Serve immediately.