Caramelize the onions by heating the olive oil in a skillet over medium heat. Add the sliced onions and cook for 20-25 minutes, stirring occasionally, until the onions are deeply browned and caramelized.
Add the grated garlic and cook for 1 minute more.
Stir in the balsamic vinegar and maple syrup (if using) and cook for 2-3 minutes until the liquid has evaporated.
In a bowl, mash half of the chickpeas with a fork or potato masher. Add the remaining whole chickpeas, caramelized onions, plant-based yogurt, vegan mayo or tahini, nutritional yeast (if using), lemon juice and zest, and chives. Stir to combine.
Season with kosher salt to taste.
To assemble the sandwiches, spread the chickpea salad mixture on sliced bread. Top with greens, cucumber slices, and any other desired condiments like Dijon mustard or plant-based cream cheese.
