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1
Bring vermouth and raisins to a boil in a small saucepan, then remove from heat and let steep until raisins are soft and plump, about 15 minutes.
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2
Cook onion in oil in a large heavy skillet over medium-low heat, stirring occasionally, until onion is tender but not browned, 5 to 8 minutes. Add chard, raisins with any remaining vermouth, 1/2 teaspoon salt, and 1/4 teaspoon pepper and cook over medium heat, turning chard constantly with tongs, until chard is tender and liquid has evaporated, about 12 minutes. Transfer to a bowl and stir in nuts, then cool.
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3
Cutting as close to bones as possible, make 1 long incision to separate meat of each rack from bones, stopping 1/2 inch from bottom (do not cut all the way through). Roll meat away from bones to create a long opening, then season inside with salt and pepper and fill with stuffing. Roll meat back over stuffing, then tie meat to bones with string (between every 2 ribs).
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4
Preheat oven to 400u00b0F with rack in middle.
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5
Stir together mustard, thyme, and rosemary and spread over both sides of each rack. Put racks of lamb in a large shallow heavy baking pan, pairing racks so that they stand up with their bones interlocking but leaving space between them at base.
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6
Roast lamb until an instant-read thermometer inserted into center of meat (do not touch bone) registers 130u00b0F for medium-rare, 25 to 35 minutes. Let stand, loosely covered, 15 minutes.
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7
Cut each rack into 4 double chops, discarding string, and serve on top of roasted red peppers.
