I like to roast the bell peppers. This is optional, you could save time and simply chop them. To roast the peppers, I place them over a gas burned and char the skin. Alternatively, you could char them in the oven under the broiler. Once the skin is charred, place the peppers in a large bowl, and cover with a towel, letting the peppers sweat and cool for 10 minutes. Then peel off the charred skin. Chop the peppers, removing the seeds and core.
In a large container with a lid, combine the chopped tomatoes, bell peppers, cucumbers, red onion, olive oil, red wine vinegar, lemon juice, salt, pepper, and feta. Stir gently. Store in the refrigerator for up to 5 days.
To serve, place two handfuls of mixed greens on a plate. Scoop some of the salad mixture on top, be sure to scoop up some of the juice that sinks to the bottom, too. Add about 2 tablespoons of tzatziki and a sprinkle of toasted garbanzo beans. Stir to combine. Serve immediately.