Heat 1 tablespoon olive oil in a 12-inch skillet over medium heat. Add the red onion and garlic and cook for about 5 minutes, until tender-crisp. Remove the vegetables to the bowl of a food processor.
Add ¼ of the garbanzo beans to the food processor, along with the lemon juice and salt. Pulse to create a smooth paste. Add the remaining garbanzo beans, bread crumbs and spinach. Pulse until coarsely chopped.
Shape the mixture into 4 (4-inch) patties. If desired, refrigerate until ready to cook.
Heat the remaining 1 tablespoon olive oil in a 12-inch skillet over medium heat. Add the falafel patties and cook until golden and crisp, turning once.
Serve the falafels on buns and top with tomato, onion and dairy-free tzatziki sauce, as desired.
Recipe by Go Dairy Free at https://www.godairyfree.org/recipes/vegan-falafel-burgers