Cover a strainer with cheesecloth and place the bulgur on top. Place the strainer in a pot filled with cold water and soak the bulgur for 10 minutes.
Pull up the sides of the cheesecloth, encasing the bulgur, and squeeze out all the water. Transfer to a large bowl.
Toss the bulgur with the lemon juice. Toss again with the parsley, mint, tomatoes, and onions and season with salt and pepper.
Stir in 1 1/4 cups of the olive oil and leave to rest at room temperature until the bulgur has absorbed enough liquid to be tender, 4 to 6 hours.
Correct the seasonings and olive oil; there should be enough that it looks shiny and moist but not gooey and oily.
Serve garnished with romaine lettuce leaves and scallions. Place a few slices of scallion in a leaf of romaine lettuce, then scoop up the tabbouleh with the lettuce leaf.
Note: the longer the bulgur sits and absorbs the olive oil, lemon juice, tomato, and onion juices, the more it will swell and dominate the salad, so keep that in mind when you prepare this salad. Remember that the proper ratio of parsley to bulgur is about 7 to 1. Many cooks make tabbouleh with a food processor by pulsing in short bursts, although I still prefer the texture of the labor-intensive method of hand-chopping all the ingredients with a large chef's knife.