Commence by preparing the pasta shells. Cook them according to the manufacturer's guidelines until they reach an al dente texture. Post cooking, drain the pasta while ensuring the reservation of approximately ¾ cup of the cooking water. Subsequently, toss the drained pasta in a tablespoon of olive oil and set it aside for later integration.

Concurrently, as the pasta cooks, proceed with the beef preparation. In a large skillet, introduce a tablespoon of olive oil and place it over medium-high heat. Once the oil achieves a slight shimmer, incorporate the diced onions. Allow the onions to cook until translucent, typically requiring 1-2 minutes. Following this, add the ground beef to the skillet. Continue cooking while stirring intermittently until the beef is thoroughly browned, which usually takes about 7-9 minutes. Drain off any surplus oil that accumulates during this process.

Upon satisfactory cooking of the beef, integrate the baby spinach, diced tomatoes, beef broth, tomato paste, minced garlic, and all requisite seasonings into the skillet. Allow the mixture to cook until the spinach has wilted and all the ingredients have amalgamated seamlessly. Reduce the heat to medium-low, and proceed to introduce the reserved pasta water, optional heavy cream, and Parmesan cheese. Continuously stir this concoction and let it simmer, ensuring the cheese melts uniformly into the sauce and the mixture slightly thickens.

To finalize, incorporate the previously prepared pasta into the skillet, ensuring that every pasta shell is adequately coated with the beef and cheese mixture.

As a garnish, consider adding an additional sprinkling of grated Parmesan cheese, finely chopped basil, or parsley prior to serving.