Cut potatoes into 1-inch chunks and mince or smash the garlic cloves. Bring 3 cups of water to a boil in a large pot, then add the potato chunks and garlic. This allows the potatoes and garlic to cook together, which infuses the potatoes with flavor from the start. Bring back to a boil and maintain a steady simmer.

While the potatoes cook, combine the oat milk and lemon juice in a small saucepan. Let this mixture sit for about 10 minutes—the acid in the lemon juice will slightly curdle and thicken the oat milk, creating a richer texture similar to curdled cream. This gives the finished mashed potatoes better body and a more luxurious mouthfeel than using plain oat milk alone.

Continue simmering the potatoes until they're completely fork-tender, about 15-20 minutes depending on your pot size and heat level. While the potatoes finish cooking, gently heat the oat milk mixture over medium-low heat until warm to the touch—don't let it boil or simmer, as you want to preserve the creamy texture. This timing means both components are ready at roughly the same moment.

Drain the potatoes and garlic thoroughly in a colander—excess water will dilute your mashed potatoes and make them watery. Return the drained potatoes and garlic to the hot pot and immediately pour in the warm oat milk mixture from Step 2. Mash with a potato ricer, food mill, or hand masher until you reach your desired consistency. I prefer using a ricer for the smoothest, fluffiest texture, but a hand masher works perfectly well for a chunkier result.

Fold in the sliced green onions, salt, and black pepper. Taste and adjust seasonings as needed—the lemon juice from the oat milk should provide subtle brightness, but you can add more of either salt or pepper to your preference. Serve warm while the potatoes still have that fluffy, creamy texture.