How to Make It
Preheat oven to 300°. Combine first 4 ingredients and 1/2 teaspoon each salt and pepper in a bowl. Gradually whisk in 1/2 cup oil in a slow, steady stream.
Toss Roma tomatoes in remaining 2 tablespoons oil; arrange on a wire rack placed on a baking sheet. Sprinkle tomatoes with 1/2 teaspoon salt. Roast 1 hour or until tomatoes are slightly browned and skins begin to pull away; let cool. Remove tomato skins, and cut tomatoes in half horizontally; discard skins.
Preheat grill to high heat (450° to 500°). Coat corn and red onion slices with cooking spray. Place on grill rack coated with cooking spray. Cook 12 minutes or until charred, turning occasionally. Remove from grill. Cut kernels from corn; discard cobs.
Sprinkle chicken with remaining 1/2 teaspoon each salt and pepper. Grill chicken 12 minutes or until cooked through, turning occasionally; let stand 5 minutes. Thinly slice chicken across the grain.
Combine arugula and lettuce in a large bowl. Add 2 tablespoons dressing; toss to coat. Top with cucumber, cherry tomatoes, red onion, chicken, corn, roasted tomato halves, and avocado. Sprinkle salad evenly with basil leaves, and serve with remaining dressing.