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Place potatoes and 2 teaspoons of salt in a large heavy saucepan and generously cover with cold water. Bring to a boil, then reduce heat, and simmer, partially covered, until potatoes are fork-tender, about 15 to 20 minutes.
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Meanwhile, bring a pot of water to a boil, then add 3 tsp of salt, cauliflower and garlic and simmer until cauliflower is very tender, 13 to 15 minutes.
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While potatoes are boiling, heat 3/4 cup milk and 3 tablespoons butter in a small saucepan until butter is melted and milk is hot but not boiling. Keep warm off heat, covered.
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Drain potatoes and return to hot saucepan. Add milk mixture, 1/4 cup Mexico City sauce and pepper; mash with a potato masher to desired consistency. Season with salt and keep warm, covered.
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Drain cauliflower in a colander and pulse with remaining 1/2 cup milk, 1/4 cup Mexico City sauce and 3 tablespoons butter in a food processor until it’s a chunky purée.
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Stir together mashed potatoes and cauliflower mixture in a large bowl and season with salt and pepper. Garnish with chopped chives and serve. Provecho!