Cook the Spaghetti:

Boil a pot of salted water and cook the spaghetti until al dente according to package instructions. Drain and set aside.
Prepare the Beef:

Heat 1 tbsp olive oil in a large skillet over medium heat. Add the beef cubes and sear until browned on all sides. Remove and set aside.
Sauté the Aromatics:

In the same skillet, add the remaining olive oil. Sauté the onions until translucent, then add the garlic and chili slices. Cook until fragrant.
Simmer the Sauce:

Return the beef to the skillet. Add the tomato sauce and beef stock. Bring to a gentle simmer, cover, and cook on low heat for 30-45 minutes, stirring occasionally, until the beef is tender.
Combine with Pasta:

Toss the cooked spaghetti with the beef and sauce until well coated.
Serve:

Divide the pasta into bowls. Garnish with grated Parmesan cheese and fresh parsley. Serve hot and enjoy!