You can use pretty much any lentil you can buy. Some lentils will cook up harder than others. Because the way lentils cook up are affected by acid and salt, the best thing to do is to pre-cook your lentils until they are almost to your desired consistency. To do this: add lentils to unsalted, plain water and boil until they are almost at the consistency you like them (some people like them soft, some like them to retain a bit of a harder texture). Keep in mind that different lentil varieties will have different textures when they cook up. For more info on the natural texture of your lentil, use Google Search to research the lentil you are using.
Peel and prepare garlic and onion
(Optional: if you have a blender you can put the garlic, onion and canned tomato in the blender and puree if you like)
Chop zucchini and cauliflower up into small pieces and set aside. This is added in about 30 minutes before your sauce is done (and this is also optional)
In a large slow cooker, mix together the onion, carrot, celery and garlic. Season liberally with salt and pepper.
Add the remaining ingredients and stir to combine.
Cook on low for 4 - 6 hours, or until lentils have softened and sauce is thickened to your taste.
About a half hour before your sauce is done, add in the chopped zucchini and cauliflower.