Into a 4- to 6-quart slow cooker, combine the sliced potatoes, water, garlic, and salt.
Lightly spray the top layer of potatoes with the olive oil cooking spray.
Press a sheet of parchment paper firmly onto the potatoes to cover them completely, folding down or tucking the edges as needed.
Cover and cook until potatoes are tender, 5-6 hours on low or 3-4 hours on high. When the potatoes are done, discard the parchment paper.
Use a potato masher to mash the potatoes to your desired consistency.
Fold in the olive oil and lemon juice.
Season with salt and ground black pepper, to taste then serve warm.