It's amazing how quickly things went from "I finally have enough lettuce!" to "Holy crap, that's a lot of lettuce." The pond looks inundated by pretty weeds, which is what I get for planting mesclun instead of something more mainstream. The smaller ones in the front are quickly catching up.
Among other searches, I looked up cooking the leaves. After all, you cook spinach, dandelion, and other greens. Why not a tender variety of lettuce? There were mixed results, but most agreed that you shouldn't cook them very long or they would get mushy. I also found this to be the popular solution to the problem of the giant Costco packages of mesclun that never get finished in time unless there's a party.
*8 C mesclun greens (or part something else)
*1/4 C diced onion
*1 clove garlic, minced
1/4 tsp salt
1 Tb olive oil
1 Tb balsamic vinegar
1. Wash greens thoroughly. If they're older and slightly wilted, soak them in ice water for half an hour. It should reinvigorate them. Cut greens crosswise in a chiffonade. It's ok if the pieces are a little big, just cut through the stem several times per leaf.
2. Drizzle oil in a large skillet. Add onion and garlic and cook on medium to soften, about 5 minutes. If part of the greens are tougher (I used some beet greens for almost half of my mix), start cooking them first. Once they are almost done, add the mesclun shreds. Sprinkle salt over greens and cook until mostly wilted, 2 to 3 minutes. Remove from heat immediately. Drizzle with vinegar and serve hot.