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Remove chard stems and thick veins from chard leaves, and cut stems crosswise into 1/2-inch pieces. Cut leaves crosswise into 3/4-inch-wide ribbons. Set aside stems and leaves separately.
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Melt butter in a 12-inch stainless steel skillet over medium-high. Add onion and 1 teaspoon salt; cook, stirring often, until onion is lightly browned, 5 to 7 minutes. Add chard stems, garlic, ginger, and garam masala; cook, stirring constantly, until fragrant and well combined, about 2 minutes. Add Madeira; cook, stirring occasionally, until liquid reduces to about 1/4 cup, 8 to 10 minutes.
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Gradually add chard leaves to onion mixture in large handfuls, tossing and stirring until leaves begin to wilt before adding another handful, about 2 minutes total. Stir in sultanas and remaining 3/4 teaspoon salt. Reduce heat to medium-low; cook, stirring occasionally, until chard leaves and stems are tender, 2 to 3 minutes. Season with additional salt to taste.
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Using a slotted spoon, transfer chard mixture to a platter. If desired, reserve cooking liquid in skillet for serving separately. Sprinkle chard mixture with sliced almonds, and serve immediately.