Heat oil on sear setting in crockpot and add onion plus 1 tsp of salt. Cook until onion starts to soften, stir in garlic. Cook 2 minutes more until garlic smells fragrant.
Add beef, breaking it up into chunks with a spoon, and season with another 1/2 tsp of salt. Cook until well browned (this step can take a while, you can skip it if short on time, but it does add a lot of flavour).
Stir in tomato paste to evenly coat everything and cook until it the paste turns a shade darker and smells toasty, about 5 minutes. Stir in eggplant (if using), canned tomatoes and salt (if using eggplant, add 1 1/2 tsp more salt, if skipping it, add only 1/2 tsp).
Close crockpot lid and cook on high setting for 4 hours or on low setting for 6 hours (longer cooking is perfectly fine if you've got more time!).
Taste sauce and add more salt by small pinches as needed until it tastes bright and flavourful.
Notes
If you use salted canned tomatoes instead, reduce the salt to 2 tsp total.