Heat 2 tablespoons of olive oil in a large skillet over medium heat until shimmering. Add the diced onion and sauté, stirring occasionally, until it becomes translucent and soft, about 5 minutes.

Heat the skillet over medium heat and add the minced garlic, stirring continuously to avoid burning. Cook for about 1 minute until the garlic releases its aroma and achieves a light golden hue, indicating it is ready for the next step.

Add the sliced bell pepper to the skillet, stirring occasionally over medium heat. Cook for about 4 minutes, allowing the pepper to soften slightly while maintaining a bit of crunch. This process enhances the dish with a lively color and a pleasant, crisp texture that complements the other ingredients.

Add the leftover cooked pasta to the skillet over medium heat. Stir gently to mix with the vegetables, ensuring the pasta is well-coated with the oil and vegetable mixture. Cook for 3-4 minutes, stirring occasionally, until the pasta is heated through and the vegetables are tender.

Turn the heat up to medium-high and add the halved cherry tomatoes to the skillet. Stir occasionally, cooking them until they begin to soften and their skins start to wrinkle, releasing their juices. This should take approximately 3 minutes, and you'll notice a slight sizzle as they cook.

Introduce the fresh spinach to the skillet, stirring continuously over medium heat until it wilts and blends seamlessly with the pasta, approximately 2 minutes. Ensure the spinach maintains its vibrant green hue, signaling it is tender yet not overcooked, and avoid any browning or excessive softening.

Sprinkle salt and freshly ground black pepper over the pasta mixture, stirring well to combine. Taste and adjust the seasoning as needed, ensuring the flavors are well-balanced. Continue cooking over medium heat, allowing the ingredients to meld together, until the pasta is heated through and the aroma is enticing.

Pour the freshly squeezed lemon juice evenly over the pasta, ensuring it is well distributed. Stir gently to incorporate the juice thoroughly. The citrus will add a refreshing note, enhancing the overall aroma and balancing the richness of the other ingredients. Serve immediately for best results.

Sprinkle grated Parmesan cheese evenly over the skillet, allowing it to gently melt into the pasta. This will create a creamy texture and enhance the savory notes. Keep the skillet on low heat for about 2 minutes, ensuring the cheese melts without burning, and the pasta is warmed through.

Serve the pasta skillet hot, garnished with additional Parmesan cheese or fresh herbs, if desired. Enjoy the contrast of textures and flavors in each bite.