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1
Preheat the oven to 375u00b0 F.
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2
Set a large pot of water over high heat. Add 1 1/2 tablespoons salt, plus more to taste, until it's very salty. Bring to a boil. Add the kasha and adjust the heat to establish a steady boil. (The method here is exactly like pasta.) Cook until the kasha is just tender-we don't want it mushy-10 to 15 minutes. Drain in a fine-mesh sieve.
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3
Meanwhile, set a very large skillet on the stove over medium heat. Add 1/4 cup olive oil. When it's shimmery, add the onions. Season generously with salt, to taste, and 1 pinch black pepper. Saute-stirring and lowering the heat as needed-until caramelized, about 40 minutes.
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4
While that's going, roast the mushrooms. Add the mushrooms to a rimmed sheet pan. Dress with the remaining 3 tablespoons olive oil. Sprinkle with salt, to taste, and 1 pinch pepper. Toss. Roast for about 30 minutes, until deeply browned.
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5
Set a large pot of water over high heat (save a dish and reuse that kasha pot!). Add 1 1/2 tablespoons salt, plus more to taste, until it's very salty. Bring to a boil. When the onions and mushrooms are almost done, add the pasta to the water. Cook until just al dente, about 8 minutes. Use a spider or slotted spoon to transfer the pasta to the skillet with the onions-this way you reserve that pasta water.
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6
Add the mushrooms, kasha, and garlic to the skillet. Toss. Add a big splash of pasta water, to loosen everyone up, plus a generous pour of olive oil-and don't be shy with either. Season with more salt and pepper, to taste. Add most of the herbs and toss. Sprinkle the remaining ones on top.
