Rinse all vegetables under cold water to remove any pesticides or bacteria.
Spin lettuce dry, and cut to your desired size.
Remove the papery skin from your red onion, and slice thinly.
Halve the grape tomatoes, or cut larger varieties into wedges.
Peel, and slice the cucumbers into 1/4-inch rounds.
Cut off the ends and tips from your celery stalks, and slice them into 1/8-inch slices.
Peel, and slice your carrots into rounds or grate them.
In a large bowl, combine the lettuce, carrots, celery, cucumbers, and tomatoes.
Plate the mixed salad and top with red onion slices and croutons.
Wait to add the dressing until ready to serve to avoid wilting the lettuce.