Sounds weird, but is actually great! Use short ferment times.
1 TBSP kosher salt (or 1/2 TBSP sea salt)
1 quart of filtered water
1 head romaine lettuce
Mix the salt and water to dissolve. Cut the romaine into half-inch slices. Cut the tomatoes into halves or quarters. Cut the onion into slices. Layer all the vegetables into a large glass container. Pour brine over everything. Use something to keep the vegetables under the brine. This depends on your container, a plate, another glass jar, some clean weights, etc. Cover the top with a dishtowel and a rubber band to keep out anything, but allows airflow. Allow to sit at room temperature for 3-5 days. Remove vegetables from the brine and allow to drain a bit. Further chop the tomatoes and onions for a salad, as desired. Eat as a salad or add to crostini, or a sandwich. A little dressing, like Ranch, or Caesar can be mixed in. Because everything is wet, a little dressing will go a long ways.
Be sure to taste the brine. If it tastes good, and is not too salty, save it in the fridge to drink, or add to salad dressings, it is also full of great probiotics!