Cook the pasta al dente according to package instructions. Drain, transfer to a large serving bowl, and toss lightly with oil to coat. Set aside.

Heat a layer of oil in a large nonstick skillet over medium-high heat. Add the onions, season with salt and pepper, and stir. Cover, reduce heat to low, and cook for 10 minutes.

Uncover, stir again, and cook 10 more minutes, or until softened and deep golden brown, stirring occasionally. Stir in the red pepper flakes, if using.

Meanwhile, in a small pan, bring the water to boil.

While the water heats, combine the kasha and egg in a medium saucepan over medium heat. Cook two to three minutes or until the kernels are dry and toasted, stirring constantly. Season with salt and pepper, then stir in the boiling water. Cover, reduce the heat to low, and simmer about eight minutes, until the liquid is absorbed.

Add the kasha mixture to the pasta and stir to incorporate. Toss in the onions with a little more oil, if desired. Season to taste and serve warm.