In a saucepan bring the chicken stock to a boil with the buckwheat. Reduce the heat and simmer, uncovered, for 15 minutes.
Bring pot of lightly-salted water to a boil. Add pasta and cook until al dente, 8 to 10 minutes; drain and rinse with cold water.
Pour the oil in a frypan over medium heat. Cook and stir the onions in the oil until lightly browned, 7 to 10 minutes. Stir the drained pasta and buckwheat into the onions and season to taste with salt and pepper.