Start by picking through your lentils. Some times, but not all, you will find little stones or other artifacts. Remove those. Then strain the lentils.
Next, add the onion, carrot, celery, and garlic to a food processor. If you do not have one, simply chop all of those ingredients very finely. Pulse until they are finely chopped.
Get a large pot onto the stove on medium, to medium high heat. Add the olive oil, warming it through.
Scrape all of the vegetable mixture out and into the pot. Cook for about 5 minutes, sweating all of the vegetables.
After five minutes, add the curry powder, salt, and pepper, and give this a good stir. I'm a big fan of cooking the seasoning for a few minutes.
After a few minutes, add the strained lentils to the pot, and give them a good stir. Add in the lemon juice. Cook for a few more minutes, then add your stock or water. Your goal is to add enough water to cover the lentils by a half inch or so. As the lentils cook, they will absorb the stock or water, so keep an eye on it as they cook, and add more stock if necessary. You are not making a soup, nor do you want them to completely absorb all of the liquid.
Bring to a boil, then reduce the heat, covering, and let simmer for about 1 hour, taking a peak 30 minutes into the cooking to adjust any liquid.
After an hour, taste the lentils and make sure they are nice and tender and that your liquid ratio is going well. If they are still a bit hard, cook longer. I like the lentils to be al dente, not mushy.
When you are ready to serve, you can either serve family style, or into smaller bowls. Garnish with fresh cilantro.