Cook pasta according to package directions. Meanwhile, saute onions and mushrooms in oil in a large skillet until lightly browned, about 9 minutes. Remove from pan and set aside., Combine buckwheat groats and egg in a small bowl; add to the same skillet. Cook and stir over high heat for 2-4 minutes or until buckwheat is browned, separating grains with the back of a spoon. Add the hot broth, salt and pepper., Bring to a boil; add onion mixture. Reduce heat; cover and simmer for 10-12 minutes or until liquid is absorbed. Drain pasta; add to pan and heat through. Sprinkle with parsley.