Step 1

Heat the oven to 375° F.

Step 2

Set a large pot of water over high heat. Add 1½ 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, stirring occasionally, for 10 to 15 minutes, until the kasha is just tender—we don't want it mushy. Use a slotted spoon or fine-mesh sieve to transfer the kasha to a bowl. Keep the water warm on the stove.

Step 3

Meanwhile, heat a very large skillet over medium heat. Pour in ¼ cup of the oil. When it’s shimmery, add the onions; season generously with salt and 1 pinch black pepper. Cook, stirring and lowering the heat as needed, for about 40 minutes, until caramelized.

Step 4

While that's going, roast the mushrooms. On a rimmed sheet pan, arrange the mushrooms. Drizzle with the remaining 3 tablespoons of the oil; season with salt and 1 pinch of pepper. Toss to combine. Roast for about 30 minutes, until deeply browned.

Step 5

Return the pot of water to a boil. When the onions and mushrooms are almost done, add the pasta to the water. Cook, stirring occasionally, for about 8 minutes, until just al dente. Using a spider or slotted spoon, transfer the pasta to the skillet with the onions—this way you reserve that pasta water.

Step 6

Add the mushrooms, kasha, and garlic to the skillet and toss to combine. Add a big splash of pasta water to loosen everyone up, plus a generous pour of oil—and don't be shy with either. Season with salt and pepper. Add most of the herbs and toss to combine. Sprinkle the remaining herbs on top.