In a small saucepan add the lentils to about 1 1/2 cups of water. Bring to a boil, then turn the heat down to med-low and cover. Cook for about 15 minutes or to taste, but don't cook so long they lose their shape. Drain and cool.
In the same pan (no need to wash it), add the rice to about 2 cups of water. Add a good pinch of salt and bring to a boil. Turn the heat down to med-low and cover. Cook for about 35 minutes, then turn off the heat and let the rice rest in the pot, on the stove for about another 5 minutes. At this stage I usually find that some, but not all the grains have split open. I like the soft crunch at this stage, but if you prefer all the grains open, by all means let it rest for an additional 5-15 minutes. Drain and cool. (I suggest cooking the rice and lentils in the morning, or even the day before if you like, and keep them in the fridge until you're ready to proceed).
Put the rice and lentils into a large bowl. Chop the celery, pepper, tomatoes, and cucumbers into smallish chunks (bigger than 1/4" but less 1/2" works for me), add to the cooled rice and lentils. Add the green onions, herbs, feta, and lemon juice and give the salad a good toss. Add the freshly grated carrots, the olive oil, sugar and a good grinding of black pepper. Taste before you add salt as the feta often does the job! Chill for however long you have, or up to a few days.