Peel and slice the onions and set them aside.
Next, heat butter and oil in large skillet over medium heat and then add the onions. Stir in the onions until they're coated with oil, and then reduce heat to a simmer. Simmer onions for 15-20 minutes, stirring occasionally, until they're soft and translucent.
Place squash in a large pot and fill the pot with water. Cook squash over the stove on high heat until they're soft.
Drain the water and then use a potato masher or a fork to mash the squash until it's smooth.
Next, stir in the cooked caramelized onions and any leftover oil by hand, along with the salt and pepper.
Enjoy!