Preheat grill pan (or other method for cooking vegetables, see notes) on medium high heat. Once heated turn temperature down to about medium and brush grill pan with avocado oil.
Grill zucchini slices, red onion, and sliced red bell pepper for about 4-7 minutes per side until they are tender. Sprinkle with a bit of salt while they are cooking. Remove from heat to a plate and set aside. Keep grill pan hot to make paninis.
Panini Assembly: Spread 1 or more tbsp of hummus onto each of the four slices of multigrain bread. Layer two slices of multigrain bread with the following ingredients in this order: kalamata olive halves (on top of the hummus), red bell pepper slices, red onion, spinach leaves, tomato slices, zucchini, and cheese. Finally, top with the remaining two slices of multigrain bread (hummus side down over the cheese).
Brush grill pan with more avocado oil. Carefully move one of your assembled sandwiches to your prepared grill pan (or panini press). Using the back of a heavy frying pan, press the top of the sandwich down firmly for a minute or two. Check to see if it looks done (has good grill lines) and carefully flip sandwich over to cook the other side, again using a heavy frying pan to press the sandwich firmly for a couple minutes.
Once done, remove to a plate and allow to cool for a couple minutes. Add more avocado oil to the grill pan if necessary and repeat the process with other sandwich.