Mixed salad leaves have to be the simplest thing in the world to grow and are actually better grown in a container because they’re safer from all the slugs and snails.
You don’t need a particularly deep container for growing baby leaves. The leaves in the image were actually the free seeds from the BBC Dig In project (which you should definitely sign up for if you’re just starting out). I sowed them in the window box and ended up with a lovely lot of leaves which my other half regularly snipped off for salads. I forgot to thin them out though, so they didn’t grow as well as they ought.