Scrub the potatoes, peel them, and cut them into chunks. Place them in a large pot of water and bring to a boil. Cook for 12 minutes.

If you are using fresh corn on the cob, remove the leaves, silk of the corn to expose the corn kernels. and discard the leaves and silk or hairs of the corn. Place the corn in a bowl, and while holding the corn with one hand upright, take a sharp knife and cut along the length of the corn to remove the kernels.

Add corn, peas, spinach, and garlic to the boiling potatoes and cook for 3 minutes. Drain the vegetables using a colander.

Transfer vegetables to a bowl, add salt. Mash vegetables using a potato masher or food processor and pulse until the mixture is combined.

Heat oil in a medium skillet over medium heat, add the onion, and cook until soft and golden brown about 3 minutes. Stir into the mashed potatoes and serve.