In a large skillet over low heat, melt 1 tablespoon of butter. Add the diced yellow onions and cook for about 5 minutes, stirring occasionally, until they become translucent and fragrant.

Increase the heat to medium-high and introduce the ground sirloin to the skillet. Season it with salt and ground black pepper. Cook for about 5-7 minutes, breaking it apart with a spatula, until the beef is browned.

Stir in the diced tomatoes and chopped celery, mixing well. Cover the skillet and reduce the heat to medium-low, allowing the mixture to simmer for approximately 15-20 minutes.

Once the celery is tender, remove the skillet from heat and add the remaining tablespoon of butter, stirring until it melts completely.

While the sauce is simmering, cook your chosen spaghetti according to package instructions until al dente. Drain the pasta and serve the rich sauce over it.