Preheat oven: Set oven to 375°F (190°C).

Cook spaghetti: Bring a large pot of salted water to a boil. Cook spaghetti until al dente according to package instructions. Drain and rinse briefly with cold water to stop cooking. Set aside.

Sauté garlic: In a skillet over low heat, melt butter with olive oil. Add minced garlic and sauté until fragrant but not browned.

Mix sauce: Squeeze lemon juice into the skillet. Remove from heat, stir in sour cream, lemon zest, and kosher salt. Taste and adjust salt as needed.

Combine pasta and sauce: Pour lemon sour cream mixture over drained spaghetti in a large bowl. Toss thoroughly to combine.

Bake: Transfer pasta mixture into a greased or oven-safe baking dish. Cover tightly with foil. Bake covered for 15 minutes, then remove foil and bake an additional 7 to 10 minutes until the top is lightly golden and pasta remains creamy.

Finish and serve: Remove from oven. Squeeze extra fresh lemon juice over the dish. Top generously with grated Parmesan and chopped parsley. Finish with another squeeze of lemon juice before serving warm.