Step 1Sort over the lentils to remove any stones. Rinse well in a sieve under cold running water and set aside.
Step 2Heat the olive oil in a heavy soup pot over medium-high heat. Add the onion, carrots, celery, garlic, cumin, oregano and bay leaf. Season with 1 teaspoon salt and cook, stirring often, until the vegetables are wilted but not limp, 8 to 10 minutes. Add the lentils; stir to mix well.
Step 3Strain the liquid from the tomatoes into the pot. Cut the tomatoes into rough dice and add to the pot. Add 4 cups water and mix well. Bring to a boil. Reduce the heat to a simmer and cook, stirring often, 15 minutes, or until the lentils start to soften.
Step 4Add the chorizo, return to a simmer and continue cooking and stirring until the lentils are soft but not mushy and the mixture is stewlike, 20 to 30 minutes longer (depending on the age of the lentils). You may need to add a little more water periodically. Season with more salt if needed and pepper to taste. Serve hot.