Place the beef tendon in a slow cooker and add just enough water to cover. Cook on low heat for 6 to 8 hours until tender. Remove the tendon, allow it to cool, and chop it into small chunks.

Using a blender with multiple blades, pulse the tendon pieces until they resemble grains of rice. If you don't have a suitable blender, finely chop the tendon with a knife or scissors into small bits.

Heat the tallow in a large skillet over medium-high heat. Add the ground beef, breaking it apart with a wooden spoon, and cook until browned, about 3 to 4 minutes.

Add the prepared tendon and chopped sage to the skillet, stirring everything together, and cook for an additional 2 minutes.

Stir in the pumpkin puree, beef bone broth, lemon juice, thyme, and rosemary. Continue cooking, stirring regularly, until most of the liquid has evaporated and the mixture thickens into a risotto-like consistency.

Taste the risotto and season with extra salt as needed.

Transfer leftovers to an airtight container and refrigerate for up to 4 days.