Preheat oven to 250°. Scatter the corn bread in a single layer on a half-sheet pan and bake until the pieces are lightly toasted, about 7 minutes.
If you wish to peel the tomatoes: Cut an X in bottom of each tomato and blanch in a large pot of boiling water 10 seconds. Immediately transfer with a slotted spoon to an ice bath to cool. Peel tomatoes and chop them.
Place lettuce, greens, 3 cups of toasted corn bread, onion and tomatoes to a large bowl and toss to combine.
Drizzle with buttermilk dressing, season with salt and pepper, and toss again. Serve immediately.
Do ahead: If you’re making this for a picnic or pot-luck, keep the croutons in one container, the dressing in another and the salad mixture in a third; this is best freshly assembled, or in the 30 minutes after.
