I make mine without any oil. The vegies "sweat" and release moisture hat they gently cook in.

Chop onion, throw into large pot on very low heat.
Chop 2-3 celery stalks, chuck in with the onion
Chop 2 carrots, chuck in pot.
Chop whatever other slow vegie you are using (cabbage, cauli, pumpkin, turnip, whatever...) Throw in pot.
Add vegie stock, enough to cover. Also add tomatoes if you're using them.
Chuck in lentils, I usually use at least a cup. Up the heat, bring to the boil.
Simmer for 30 mins (longer for brown lentils)
Add pasta
Add any of your "quick" vegies (sliced green beans, peas, spinach etc)
After 10-15 minutes, remove from heat, serve.

Deeeeeeeeeeeelicious.