Thanks, Port and Lemon!
Here is how I make tarka daahl, but it is probably very inauthentic.
Take one big heavy pot and one any-old -saucepan. In the any-old-saucepan, put red lentils and enough water to cover them and, say, an inch to spare. Stir, heat, and leave to simmer. Throw in a bay leaf if you remember.
Meanwhile, in your heavy pot, light the gas under it and toast your whole cumin, coriander, fenugreek and any other seed type spices you fancy - mustard seed etc. When they smell great and toasty tip them out into a pestle and mortar and crush them together. Then heat oil in your pan - I use sunflower oil - and fry chopped garlic, fresh chilli and onion, maybe some ginger. When they are soft add your pestled spices and some turmeric. By this time the lentils are probably cooked and fluffy. So throw them on top of the spices in the heavy saucepan, add any soggy tomatoes you need to find a use for, finely chopped, and put the lid on. Add more salt than you think.
You can cook the rice in the same saucepan you boiled the lentils in, white basmati rice will be done in a flash.
You should finish the daahl with oil or butter in which you have fried garlic, and chopped fresh coriander. I do not always get around to this.
Apologies to any real Indian food experts who are appalled by this slap dash approach. I can't remember which of this way of doing it came from a recipe and which I totally made up.
I think in a slow cooker maybe you would still need to fry the onion and spices, tip that in with water and lentils and then leave it?