.....  In the real world, we nobody counts lentils. But that isn't the point. We aren't talking about the real world. We are talking about <em>grammar</em>. And, grammatically, 'lentils' are countable, simply because the word 'lentils' is PLURAL. It has an 's' on the end. There is a singular form - a lentil - and there is a plural form 'lentils'. That's all there is to it.

Just remember this simple rule:

If a word is plural, it takes plural verbs, plural pronouns, and the quantifiers 'many' and 'few/fewer':

Lentils are good for you. They are full of protein and fibre. Eat as many lentils as possible.

If a word is singular, it takes singular verbs, singular pronouns, and the quantifiers 'much' and 'little/less':

Chocolate is bad for you. It is full of sugar and fat. Eat as little chocolate as possible.

Forget about 'countable' and 'uncountable' - just think about singular and plural.

Hello, Rose. Are lentils countable? Yes, they are.

I like lentils, and I want to propose a much, much simpler way of looking at this:

Many students, especially Russian speakers, get very confused and stressed about the idea of countable and uncountable nouns. They imagine that it is some kind of great philosophical question, or some kind of innate scientific property of the objects themselves that makes them countable or uncountable. It is neither of these things. In fact, this question isn't even anything to do with literally counting objects, 1, 2, 3 and so on. I don't blame learners for getting frustrated by the idea of countability. Why is 'money' uncountable, when everybody counts money? And why are 'lentils' countable - which they are - when no sane person would ever count lentils? As the answerer above has said, counting lentils is a pointless thing to do, and we'd normally quantify them by saying 'a packet of lentils' , '500g of lentils' and so on.

To be continued.....

I agree. If you're talking about lentils for eating, you'd be measuring them, not counting them. They're very small and eaten in quantities large relative to their small size. Counting lentils you eat would be like counting peas. Probably not going to happen. You would be discussing something like this in terms of an amount, not a number. 

ex) 1 cup of lentils, 100 grams of lentils

Same thing with beans^^ An amount, not number is most likely what you would need to use. If you really needed to count them individually, I would just say "5 peas", "22 kidney beans", or "63 lentils" or maybe 'lentil seeds". I don't really talk about lentils so I'm not sure^^ I think both would be correct though.