The problem is that a lot of substitutions are based on what you have on hand.
For instance, I have both liquid smoke and smoked paprika, both of which would add that smoky quality that people associate with cumin.
We also need to consider what other ingredients are going into the dish.
I have spice blends that contain cumin in varying degrees (garam masala, taco seasoning, a rub for ribs), and if they contain other ingredients that I might be using anyway, I'd consider replacing all of them with the spice blend.
cooking.stackexchange.com/users/67/Joe
Chili powder and things like chili or taco spice mixes will normally have cumin as an ingredient.
Of course they have other ingredients as well, but they might work well in a lentil soup.
cooking.stackexchange.com/users/613/Kenster
In India, when we make lentils we finish it by tempering with cumin or black mustard seeds.
Infact these two are used interchangeably with many other dishes and gives the right kind of flavour required for any such dishes.
Incase
you don't have even that on hand, flavour your dish with othe spices like chilli powder and garam masala and it should turn out just fine.
Many spice mixes contains cumin powder so if you could get your hand on any of the mixed spice powders, it should do the trick.
cooking.stackexchange.com/users/27535/Neels
Coriander might give you the right general sort of flavor, but I think it's darker and heavier than cumin
(I'm struggling to describe this well, obviously).
Perhaps mixed with paprika to brighten it up a little?
If you've got some, give it a sniff and see; it might work for the desperation, if nothing else.
cooking.stackexchange.com/users/17272/Erica
My first instinct was curry powder and perhaps some garam masala.
Not really like cumin, but also a classic combination with lentils and onions - an indian twist, so to speak.
(Amp up the heat with chili and top with a dollop of yoghurt, if you like.)
cooking.stackexchange.com/users/28879/Stephie
I've just run out of cumin and rather than go to the shops for more I wonder what I could substitute for cumin?  I'm making a lentil soup, which calls for sweating onions and the finishing the onions with cumin before adding the stock.  What can I use instead for variety or desperation?
What can I use as a replacement for cumin?
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<substitutions><spices>
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cooking.stackexchange.com/users/619/Johnny Maelstrom
This depends on the spices you have and the recipe you will be following. Many spice mixes contain cumin, so consider adding a compatible one like garam masala, taco seasoning or spice rubs rather than the other spices in the recipe. You can also try coriander, but the flavour is quite distinct.  The choice of a substitute for cumin depends on availability and the dish being prepared. Many spice mixes contain cumin so they could be a good choice. Another option is coriander.
Finding a substitute for cumin depends what you have available and what ingredients are going into the dish. Many spice mixes contain cumin and therefore could be a good substitute. Coriander may be a suitable choice.