Peel the onion and carrots, then finely chop them. In a large skillet, heat a little oil and sauté the onion and ground beef for a few minutes. Add the carrots, bouillon cube, and crushed tomatoes. Let simmer until the sauce has reduced, then season to taste.

Mash the drained potatoes and butternut squash with the milk and a generous pat of butter. The mash should be creamy; season with salt and pepper to taste.

Use a fork to make decorative lines on top, dot with a few small pieces of butter, and bake for 30 minutes until the top is golden brown.