Add all ingredients to the slow cooker. Cook on low for 8 hours or on high 5-6 hours, until the lentils are cooked through and the mixture thickens. Open the slow cooker and, using a potato masher, mash about 1/3 of the filling. Give it a good stir, replace the lid, and let it cook for another 30 minutes or so until thickened.
Meanwhile, toast the buns. Brush insides with olive oil and place cut-side-up under the broiler in your oven. Broil 30 - 60 seconds until golden.
Place buns on plate and spoon filling over the top. Add toppings if desired. Serve.
Leftover filling keeps well in an airtight container in the refrigerator for 2-3 days.
Use gluten-free buns or you can go the bunless route - the filling is also great over brown rice or spaghetti squash.