1. Begin by placing the diced lamb in a pot. Add enough water to cover the lamb, and bring to a gentle simmer over medium heat. Cook the lamb for approximately 20 minutes until tender.
2. While the lamb is cooking, prepare the vegetables. In a separate pot, add the diced beets and squash. Cover with water and bring to a boil. Reduce the heat and let simmer for about 15 minutes or until both the beets and squash are soft.
3. Once the lamb is cooked, remove it from the pot and set it aside. Reserve some of the cooking liquid for blending if needed.
4. Drain the beets and squash, and add them to a blender alongside the cooked lamb and navy beans. Blend until smooth, adding reserved cooking liquid or fresh water/stock as needed to achieve the desired consistency.
5. Taste the puree to ensure it is smooth and at an appropriate temperature for serving. If needed, you can blend it a little longer to achieve a silky texture, ideal for babies.
6. Serve the puree immediately or let it cool down before storing.

Summary:
This Beet, Lamb, Navy Bean, and Squash Puree is packed with nutrients that are beneficial for a baby's growth and development. Beetroot provides fiber and antioxidants, while lamb offers essential proteins and iron. Navy beans add additional fiber and protein, and squash is rich in vitamins A and C. This recipe is suitable for babies aged 4-10 months.

Storing Options:
You can refrigerate any leftovers in an airtight container for up to 3 days or freeze portions in an ice cube tray for up to 3 months. Before serving, always ensure the food is fully thawed and heated appropriately.

Possible Side Effects/Allergies:
Introduce each ingredient separately before combining to monitor for any allergic reactions. Particularly watch for reactions to lamb or navy beans, as they may be uncommon allergens for some babies.