Cook the whole wheat spaghetti according to package instructions in a large pot of salted boiling water until al dente. Drain and set aside, reserving 1/2 cup of the pasta water.

In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté for about 3-4 minutes until translucent.

Add the minced garlic to the skillet and cook for an additional 1-2 minutes until fragrant, being careful not to burn it.

Add the lean ground beef to the skillet, breaking it up with a spoon. Cook until browned and cooked through, about 5-7 minutes. Drain any excess fat if necessary.

Stir in the cherry tomatoes, spinach, oregano, salt, black pepper, and red pepper flakes (if using). Cook for another 3-4 minutes until the spinach is wilted and the tomatoes are softened.

Add the cooked spaghetti to the skillet along with the reserved pasta water. Toss everything together until well combined and heated through.

Serve hot, topped with grated Parmesan cheese.