We have Cornwall Microgreens on site, so are blessed with rocket fuel baby greens year round. We generate a lot of our own electricity, thank goodness.
Thinking back to my solo gardening days, one of the winter crops I adored was winter savory. It's a tiny leaf but packs a lot of flavour. Nothing better to pep up an omelette or scrambled eggs, but also adds a lovely edge to any salad. Other perennial herbs can go in the salad too - rosemary, sage, thyme, lavendar for instance. They don't add much bulk but a lot of interest, both colour and flavour.
As to winter salads generally, I add onion, carrot and beetroot anyway, and if short on bulk then yes, I use cabbage and apple too. And if you grate carrot, beetroot and swede / turnip together, it's nutritious, tasty and bulky. Mix some apple and raisins in, with a dressing if that's your style.
I've been known to use grated courgette to replace cucumber when cucumbers are in short supply, tinned (or frozen/thawed) sweetcorn to add variety in texture and colour, a sprinkle of seeds, nuts and raisins/sultanas, chard stems are a great replacement for celery and will keep growing for a lot of the year in a polytunnel, leaf beet and spinach have a long season too under cover, if kept picked and pruned so they don't get to flower. (Obvs we get longer growing seasons in Cornwall than you'll get north of the border.) A pot of basil on an indoors windowsill adds leaf, colour and flavour.
That last has had me wondering whether a very small indoor plant nursery could provide salad greens year round...

Or even do your own micrgreens (on a small scale) in the kitchen...
Oh! And of course, sprouting seeds/beans. A fantastic salad ingredient, easy to make enough for plenty salads for a household in a sprouter in the kitchen. I'd forgotten about them, I used to eat loads of sprouted things year round before I lived alongside Microgreens.