How to Make It
To make sauce, place walnuts in a bowl; cover with boiling water. Let walnuts soak 15 minutes. Drain, and rub walnuts with a clean dish towel to remove as much walnut skin as possible. Discard skins.
Place walnuts, cheese, vinegar, sherry, sugar, salt, and 1/3 cup milk in a blender; process until smooth. (Sauce will be very thick, more scoopable than pourable.) Stir in remaining milk, 1 tablespoon at a time, to thin sauce, if desired.
To make chiles, preheat broiler to high with oven rack 6 inches from heat. Place chiles on a baking sheet; coat chiles with cooking spray. Broil for 10 minutes or until blistered on all sides, but not completely charred, turning after 5 minutes. Place blistered chiles in a bowl; cover tightly with plastic wrap. Reduce oven to 400°F.
Bring a small saucepan of water to a boil over high. Add hibiscus flowers; remove from heat. Let stand 30 minutes. Drain flowers well; coarsely chop.
Heat oil in a large skillet over high. Add onion; sauté 2 minutes or until lightly browned. Reduce heat to medium-high. Add oregano and garlic; sauté 5 minutes. Increase heat to high; stir in tomato, jicama, carrot, chipotle, salt, and cinnamon. Bring to a simmer, and cook, stirring occasionally, until liquid is absorbed, about 2 minutes. Place tomato mixture in a large bowl; cool 10 minutes. Stir in chopped hibiscus and rice. Cool to room temperature.
Remove chiles from bowl. Peel; discard skins. Using scissors, cut a slit lengthwise in each chile. (Do not cut through the opposite side of the chile.) Remove and discard seeds.
Stuff each chile with about 1 cup rice mixture. Place stuffed chiles, cut sides up, on an aluminum foil–lined baking sheet coated with cooking spray. Bake at 400°F for 15 minutes or until filling is heated through. Place 1 stuffed poblano on each of 8 plates. Top each with about 2 tablespoons walnut sauce. Sprinkle evenly with cilantro and pomegranate arils.