I've been picking salad leaves just from perennial vegetable plants to see what might go into the salad bowl without doing any annual sowing. I forgot a few prime candidates, amongst them the watercress which is growing in the backyard in pots placed in a tub of water and the mallows (I'm growing musk mallow and common mallow) which are mild in flavour and can go into the salad bowl in quantity. Of course a multitude of other leaves could have been added such as foraged lime and hawthorn leaves, self-seeded annuals like lettuce, lambs lettuce and winter purslane and wildflowers such as dandelion and jack-by-the hedge.
But this is what I did pick!
Top: giant chives, scorzonera, lovage, buck's horn plantain, turkish rocket, chives, garlic cress
Middle: pink purslane, sage, mountain sorrel, hop, sweet violet, strawberry, buckler-leaved sorrel
Bottom: lemon balm, salad burnet, red-veined sorrel, wild rocket, marjoram, sweet cicely, sea beet
N.B. I sell a range of perennial vegetable plants on my website.