How to Make It
Preheat broiler to high.
Cut bell peppers in half lengthwise; discard seeds and membranes. Place pepper halves, skin sides up, on a foil-lined baking sheet; flatten with hand. Broil 10 minutes or until blackened. Wrap peppers in foil, sealing edges. Let stand 10 minutes. Peel. Finely chop 2 pepper halves; place in a large bowl. Set aside remaining 4 pepper halves.
Reduce oven temperature to 375°.
Place mushrooms in a food processor; pulse 10 times or until finely chopped.
Heat a large skillet over medium-high heat. Add oil to pan; swirl to coat. Add onion; sauté 6 minutes or until tender. Add garlic; sauté 1 minute, stirring constantly. Add mushrooms; sauté 10 minutes or until liquid almost evaporates, stirring occasionally. Cool 5 minutes. Add mushroom mixture to chopped bell peppers.
Place chickpeas and peas in food processor (do not clean from mushrooms); pulse 10 times or until finely chopped. Add pea mixture to mushroom mixture. Add 2 tablespoons basil, 1/2 teaspoon salt, walnuts, and next 7 ingredients (through egg); gently mix until well combined. Spoon mixture into a 9 x 5-inch loaf pan coated with cooking spray. Bake at 375° for 40 minutes or until a thermometer registers 155°. Let stand 10 minutes. Cut loaf into 6 slices. Sprinkle with remaining 2 tablespoons basil.
Combine remaining 4 bell pepper halves, vinegar, and remaining 1/8 teaspoon salt in a blender or food processor; process until smooth. Serve sauce with meat loaf.