Here is my go-to source for all things vegetarian low carb - lots of ideas on what to eat: https://docs.google.com/document/d/1GOgxLJy3Txl5hdzBYfUEJlpTggNcoNnPv7pqLGya1h0/edit

Also, this sub-reddit is solely for vegetarian keto and has a lot of good info too: http://www.reddit.com/r/vegetarianketo

I just looked up lentils because I don't eat them. Per 1 serving (1 tablespoon) it's 2.5g carbs and 1g fiber, or 1.5g net carbs. For 1tblsp of food, that seems like a lot to me. They also have 0g fat and 1g protein, so in my eyes it's a more carby food than anything else.