> It would be... unusual... for an American home to contain enough space to grow the family's annual supply of lettuce

I grow leafy greens in containers, and while not totally self sufficient, it provides a decent amount. I have multiple pots that I rotate through offsetting planting so new seedlings are going in as older plants are harvesting. If you're wanting head lettuce types then it's different, but if you just want leafy greens, you can totally do more than I think you're thinking you can.