Preheat oven to 400 degrees.
Slice the summer squash half inch thick, and the red onion thinly. Toss them in olive oil, season with salt and spread out on a baking pan. Oven roast until the onions begin to char and the squash is soft, about 15-20 minutes.
In a separate baking pan, bake the Falafel balls until hot, about 8-10 minutes.
While they are cooking, smash the avocado in one bowl and the summer squash in another. Season the avocado with salt and the squash with the harissa powder.
Toast the slider rolls and spread avocado smash on the bottoms and squash smash on the tops. Add the charred onions and two Falafel balls to each slider and serve.