I don't really recommend regrowing lettuce from the bottoms of store-bought lettuce. By the point it's been harvested and put on the shelf at your grocery store, the plant would have already used up most of its energy and won't have enough left to produce leaves as big and brilliant as the first time. Romaine lettuce also takes a bit longer to reach full maturity than most other lettuce varieties, at 75-85 days, so you'd have to wait that long if you wanted a full head of Romaine lettuce. I actually recommend starting several lettuce plants from seed and harvesting their leaves as they grow so that they can continue to produce more leaves while you still have enough leaves for your salad because you're growing multiple plants. Good luck with your salads, they sound delicious!