First, cook your pasta in boiling salted water until cooked to your desired doneness. Drain and set aside.
While the pasta is cooking, let's cook that kasha. Coat kasha in the egg with 1/4 teaspoon salt and a little pepper in a bowl. Heat a medium saucepan over medium heat and add kasha. Cook while stirring until the egg disappears and the kasha is toasted and fragrant, about 5 minutes. This will prevent mushy kasha.
Meanwhile, boil 2 cups water. Pour the boiling water over the kasha with 1/4 teaspoon salt, cover, lower heat and simmer for about 15 minutes until kasha is tender and liquid is absorbed.
Remove kasha from heat, let sit for 10 minutes, then fluff with a fork.
Now, the good stuff. As the pasta and kasha are cooking, make the veggies.
Heat 2 tablespoons schmaltz in a large sauté pan over medium heat. Add onions and 1/4 teaspoon salt, and sauté for 10 minutes, stirring often, or until they start to turn golden brown. Then add 1 more tablespoon schmaltz and add mushrooms, 1/4 teaspoon more salt and sauté for 7 more minutes until everything is golden. Set aside on a plate.
Add 2 more tablespoons schmaltz and the Brussels sprouts, 1/4 teaspoon salt and sumac. Sauté for 15 minutes, or until Brussels are golden as well. Oh man we are getting close.
Now put it all together! Lightly stir together pasta, kasha and vegetables. Toss with lemon juice and zest and remaining schmaltz and salt and pepper to taste. Top with cilantro and a fried egg if desired. Serve warm or at room temperature.