At the advice of Trina from Silver Height Farm where I buy my plants from, she recommended the Pablo Head Lettuce.
I bought a 6-cell pack and got them all in there. Each cell contained a few plants that I separated out. They were planted about 3-4 inches apart.
When planting these I noticed that the soil was pretty shallow, maybe 2-3 inches at some points. This could be a reason why the onions didn’t get to be very large. At other spots the soil was about 4-6 inches.
Either way, it should be good for the lettuce. I’m hoping to be able to harvest these for a few weeks.