Kosher or sea salt and freshly ground black pepper
1/2 cup walnuts, toasted
Bring a large pot of salted water to a boil. Add spaghetti and cook until al dente, about 8 to 10 minutes. Drain, reserving 1 cup cooking liquid. Transfer spaghetti to a large bowl.
In a medium-size saute pan over medium heat, add oil and cook onion and garlic until softened, about 5 minutes. Add beets and cook another 8 minutes, until softened. Add reserved cooking liquid and goat cheese; cook, stirring, until cheese softens into a sauce. Add lemon juice; then add salt and pepper to taste.
Add sauce to spaghetti and toss well. Divide among 6 bowls and top each serving with toasted walnuts.