Place the potatoes, squash, and salt in a pot with water. Bring to a boil, then reduce the heat and cook until the potatoes are tender but not falling apart. Drain, reserving a little of the cooking water in case you need it. Remove the potatoes and keep them covered in a separate bowl so they stay warm.
In the same pot, heat the olive oil and melt the butter. Lightly sauté the garlic, then cook the parsley for 1 minute.

Add the potatoes and squash back to the pot and mash well. Adjust the salt if needed.
If the mash is too thick, add a little of the reserved cooking water to reach your desired consistency.
The squash gives the mash a smooth texture, great flavor, and a beautiful color.