Preheat the oven to 375 degrees.
Cook the spaghetti according to the package directions. Drain and keep warm.
In a skillet, heat the butter over medium heat. Once it is melted
add the onions and cook for 3 minutes, stirring constantly.
Add in the garlic and cook for 1 minute.
Remove from heat and add half of the fresh lemon juice and sour cream. Mix well until combined.
Add the cooked spaghetti, salt, pepper, and half of the Parmesan cheese. Toss to combine.
Add the lemon zest a little at a time, tasting after each addition, until you get the flavor you are looking for.
Pour the spaghetti into a 9 x 13 pan and bake for 15-20 minutes.
Remove from oven, sprinkle with remaining lemon juice, Parmesan cheese, and parsley. Serve hot.