Peel the garlic. Separate the garlic cloves and place them in a large mixing bowl. Either buy pre-peeled cloves, or check the blog post for a video on how I peel garlic cloves faster.
Cook the potatoes. Place the peeled and quartered potatoes in a large pot and cover with cold water. Season generously with salt and bring to a boil. Reduce to a simmer and cook for 15–20 minutes, or until the potatoes are fork-tender.
Soften the garlic. Meanwhile, combine the peeled garlic cloves and ½ cup butter in a small saucepan. Cover and cook over low heat for about 20 minutes, stirring occasionally, until the garlic is very soft and tender. Avoid browning the garlic.
Prepare the garlic sauce. Stir the flour into the garlic and butter and cook over low heat for 2 minutes, stirring continuously. Remove from the heat and gradually add the soy milk, salt, and white pepper. Blend until completely smooth using a blender. Return to the saucepan and simmer gently for another 2 minutes, stirring occasionally, until slightly thickened.
Mash the potatoes. Drain the potatoes thoroughly, then pass them through a potato ricer into the empty pot. Pour in the garlic sauce and gently fold together until smooth and creamy. Taste and adjust the seasoning as needed. Keep warm while preparing the topping.
Brown the butter. Add the remaining 3 tablespoons of butter to a light-colored skillet over medium heat. Allow the butter to melt and foam, swirling the pan occasionally. For more sage flavor in the butter, add the sage leaves now, while the butter is foaming. Continue cooking until the milk solids turn golden brown and the butter develops a rich, nutty aroma. Reduce the heat to low.
Fry the sage. If you haven't added the sage yet, add the leaves to the brown butter and fry for about 30–60 seconds, until crisp. Remove the skillet from the heat and immediately transfer the sage leaves to a paper towel-lined plate. Be careful not to burn the butter or sage.
Assemble and serve. Spoon the mashed potatoes into a shallow serving bowl and create swirls using the back of a spoon. Drizzle the warm brown butter over the potatoes, allowing it to settle into the crevices. Arrange the crispy sage leaves on top, crumbling a few for extra sage flavor throughout. Finish with flaky sea salt and serve warm.