Rinse the bell pepper, remove the seeds and membranes and cut the vegetable into small cubes. Peel the half red onion and mince it as finely as possible. Chop the parsley coarsely. Halve the lemon and juice it. Rinse the cooked lentils in a colander and let them drain thoroughly.
Transfer the drained lentils to a large mixing bowl. Add the bell pepper cubes and the finely minced red onion. Toss these ingredients together so the vegetables are evenly distributed.
Pour the two tablespoons of olive oil and a generous squeeze of fresh lemon juice over the lentil mixture. Season with salt and pepper to taste and stir well. Crumble the feta with your fingers over the bowl and scatter the chopped parsley on top. Toss everything together gently one more time.
Let the salad sit for a few minutes so the flavors can infuse into the lentils. Then divide the salad among the plates and serve immediately.