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If they're dried you've got to rehydrate them then cook them through.
If canned you just rinse them off and add them to shit. Soup, hipster salad, make a lentil curry with some onions and curry powder. You can make lentil burgers. 
Lentils aren't very interesting but you can use them to replace bread, pasta, potatoes or rice.
I just made a chickpea curry, costs virtually nothing, healthy, some protein