Add all the quiche batter ingredients to the bowl of a food processor. Process for 7-10 seconds until well combined. Transfer the mixture to a large bowl and set aside while you prepare the vegetables.
Heat one tablespoon of oil in a large skillet over medium heat. Once hot, add the onion and garlic. Cook for about 5 minutes, stirring occasionally until soft. Add the red bell pepper and zucchini and continue to cook for 5-7 minutes until the vegetables are cooked.
Season with salt and pepper and add the cherry tomatoes, cook for another 2 minutes.
Remove from heat, reserve 1/4 of the vegetables for topping and transfer the rest to the quiche batter.
Grease three 6.5-inch cast iron skillets (or use one larger). Divide the batter into the skillets, and top with the remaining vegetables.
Bake for 25-30 minutes, and turn on the broiler for the last 5 minutes. Let cool 5-7 minutes before serving. Top with fresh basil and/or more red pepper flakes.
Leftovers will keep in the fridge for up to 3 days. To reheat, cover with aluminium foil and warm in a 325°F preheated oven for about 25 minutes.