The eggs that became those aphids are in the soil your lettuce was planted in. Maybe, possibly, pouring boiling water on the soil would help. If plants growing in that medium have aphids again the simplest method to get rid of them (the aphids) is to knock them off the plants with a sharp stream of water.

Aphids do not lay their eggs in the soil, unless they have been gathered into an ant colony. They are deposited in tiny crooks and crannies along the stems, leaf axils, bud scales, etc., where they overwinter.

Plant lavender near anything you want aphid-free. This really works. My roses that are close to lavender have no greenfly. Those further away are covered in them. Greenfly really dislike the aroma of lavender.

Also encourage as many birds as possible to your garden. They will keep greenfly (and other pests) down. Wasps also get through a huge amount of greenfly so if there is a wasp nest which is not causing problems in your house please leave it alone and let the wasps do their bit. They will move on to pastures new in the winter.

One thing I like doing with a plant heavily infested with greenfly is tapping them off into the palm of my hand. I them empty the contents of my hand over a spider web and watch the spider suck the juices out of all those b*stards that have been sucking the juices out of my plants. Payback time.