Thanks for the suggestions, everyone! I used baby spinach, as we were snowed in and had spinach on hand. It was delicious!

Another good substitute for swisschard is fresh celery stalks and leaves.

http://casa-giardino.blogspot.com/2010/12/sustainable-eating-lentil-soup-with.html

You could always substitute beet greens if you have them. They are in the same family as chard.

hobbybaker - Another choice would be escarole.

Pierino - The Italian word of swisschard is "bietola". We grow it in our garden right here in NJ

http://casa-giardino.blogspot.com/2010/07/swiss-chard-on-steroids.html

In Europe Swiss chard goes under the name "bletes."

Collards just take so much longer to cook than chard. I think the napa cabbage suggestion is a good one, or more mature spinach.

Side note: here in South Africa, Swiss Chard is widely called Spinach. Only recently that I've been able to reliably find actual spinach, labeled here as 'baby' spinach. I'm really not wild about Swiss chard, makes my tongue go funny...

Grow chard in your garden, if you have one. It's easy and foolproof. Full of vitamins and delish in soups and sauteed in butter with a little bit of lemon juice.

Collards were my first thought also Nora.
Good call.

But they can sometimes be hard to find outside of the south.
I do like Collards, white beans and ham in soup.

Terrible not find chard! Try another store.

I suggest collards rather than kale or mustard. Collards are sweeter and milder. I would cook them separately until done, simmer with the lentils for a few minutes.

I'd probably go for something more substantial than spinach for a winter dish.

Kale, or mustard greens would be a substitute. Cook it a bit before adding to the soup. With those be aware they need to be cleaned very well, as with leeks, they can have sand on them. Some stores sell them pre-washed in bags. Still you'll need to prep them by trimming off some of the tough stems..and chopping the large leafs into shreds. Some Nappa cabbage would work also.

Thank you for the quick responses!

Yes spinach with lentils is delicious too. I make one with curry powder and tomato and lemon.

Frozen spinach (thaw it first) would be perfectly acceptable and perhaps a sturdier substitute than baby spinach.

Check this food52 recipe out for a lentil soup with spinach in it:
http://www.food52.com/recipes/2348_lentil_and_sausage_soup_for_a_cold_winters_night

Spinach is a good substitute, but doesn't need to be cooked as long as chard. I'd add it at the very end of the recipe -- stir it into the soup & cook it just 1 minute.