Open and drain the 11 oz can of brown lentils in a fine-mesh sieve; rinse under cold water and let excess water drain. Transfer lentils to a large bowl.
Peel and finely chop the purple onion.
Wash the 9 oz cherry tomatoes and cut each tomato in half.
Wash the ½ large cucumber and dice it into small pieces.
Chop the 1 Tbsp fresh mint leaves and the 2 Tbsp fresh parsley.
Add the chopped onion, halved tomatoes, diced cucumber, mint, and parsley to the bowl with the lentils.
Squeeze the juice from ½ lemon over the mixture, removing any seeds, then add 3 Tbsp olive oil.
Season with salt and black pepper to taste and gently toss everything until evenly combined.
Serve the lentil tabbouleh with the 4 pita breads.