Coat an unheated grill rack with cooking spray. Place peppers, corn, and onion on the prepared rack on an uncovered grill directly over medium heat. For onion, grill about 10 minutes or until crisp-tender, turning once; remove from grill. For corn and peppers, grill for 25 to 30 minutes or until corn is crisp-tender and lightly charred and pepper skins are charred, turning occasionally; remove from grill. Wrap peppers in foil; cool slightly.

Meanwhile, in a covered large saucepan, cook beans in lightly salted boiling water until crisp-tender. Allow 2 to 5 minutes for haricots verts or 10 to 15 minutes for green beans. Drain; submerge in ice water. Drain well; set aside.

When peppers are cool enough to handle, cut off tops and remove seeds. Carefully remove charred skin. Coarsely chop peppers, keeping red and poblano peppers separate. Reserve red pepper for Roasted Red Pepper Vinaigrette. When corn is cool enough to handle, cut corn kernels from cobs; discard cobs.

In a large bowl, combine lettuces. Add beans, corn, red onion, and poblano pepper. Pour 1 cup of Roasted Red Pepper Vinaigrette onto salad; toss gently to coat. Cover; chill any remaining vinaigrette for other salads. Makes 12 (1-cup) servings.

In a food processor, combine chopped roasted red pepper, 1/4 cup sherry vinegar or white wine vinegar, 2 tablespoons water, 1/4 teaspoon salt, and 1/8 teaspoon ground black pepper. Cover and process until smooth. With the processor running, slowly add 1/2 cup olive oil in a steady stream. Cover and chill until serving time.

Cook vegetables as directed in Steps 1 through 3; cover and chill for up to 24 hours. Prepared Roasted Red Pepper Vinaigrette as directed; cover and chill for up to 5 days.