Put the quartered potatoes in a large pot and fill it with enough cold water to cover the potatoes by one inch. Add 2 teaspoon salt. Bring to a boil over high heat, then reduce to a simmer and cook until the potatoes are fork tender, 12-15 minutes
Drain potatoes in a colander. Immediately return the drained potatoes to the still-warm pot. Put the pot over low heat and shake until the potatoes are dry. Remove from heat.
While potatoes are cooking, heat 2 tablespoon butter in a large skillet over medium-high. Add the garlic, leeks and cabbage. Cook, stirring, until the cabbage is softened; about 5 minutes.
Heat the milk and butter together until the butter is melted and the milk is hot but not boiling.
Add the potatoes, milk and butter to the cabbage mixture and mash until potatoes are smooth. Add salt and pepper to taste and stir so that everything is combined.
To serve, make a well in the top and add more melted butter if desired. Garnish with scallions and serve hot.
To prep this recipe ahead: peel and quarter potatoes and cover with water until ready to use. Shred the cabbage and slice the leeks. Keep everything refrigerated until ready to cook. If you are using a potato ricer or a food mill to mash the potatoes, do this before adding them to the cabbage mixture along with the milk and butter.Shred or slice the cabbage fairly thin for this recipe's cook time. If you prefer to have chunkier cabbage, you will need to sautee it for a longer time to soften it. If you are using salted butter, take and adjust when adding salt.You can replace the milk with ½ cream or half & half for a richer dish, or buttermilk to add some tang.