How to Make It
Preheat oven to broil with oven rack in middle of oven. Rub bell pepper halves and 2 onion quarters with 1 teaspoon olive oil. (Reserve remaining onion quarters for another use.) Arrange bell pepper halves and onion quarters, cut sides down, on a rimmed baking sheet. Broil in preheated oven until bell peppers are well charred, about 15 minutes, adding garlic to baking sheet halfway through broiling. Place bell peppers, onion, and garlic in a bowl, and cover with plastic wrap. Let stand at room temperature until vegetables are cool enough to handle and bell peppers are softened, about 10 minutes. Remove and discard garlic skins and charred skin from bell peppers and onion.
Transfer bell pepper mixture to a blender; add walnuts, Thai chile, lemon juice, salt, and remaining 2 1/2 tablespoons oil. Begin processing on low speed, and gradually increase speed to high, processing until smooth, about 45 seconds total. Set bell pepper puree aside.
Heat a large heavy-bottomed saucepan or Dutch oven over medium-high until very hot, about 3 minutes. Add mussels, and shake pan to arrange mussels in an even layer. Pour wine into pan, and cover with lid. Cook, shaking pan occasionally, until mussels begin to open, about 5 minutes. Stir in harissa and butter. Cover and cook, shaking pan occasionally, until mussels are completely open, about 2 minutes. Remove from heat, and discard any mussels that did not open. Stir in bell pepper puree. Using a slotted spoon, transfer mussels to deep bowls, and pour sauce from pan evenly over mussels. Sprinkle evenly with Thai basil leaves and pickled red onion petals.
Harissa is available in both mild and spicy versions, with varying levels of salt. Taste the harissa before using to adjust the heat in the broth.