1. Preheat the oven to warm, or preheat a warming drawer. Sauté the leftover vegetables in a nonstick skillet in 2 to 3 tablespoons of water for 3 to 5 minutes to heat through. Stir occasionally, and add water as needed to prevent burning. When the veggies are heated through, remove the skillet from the heat, drain off any excess water. Transfer the veggies to a glass bowl; cover to keep warm. Wipe out the skillet, and set it aside.
2. In a separate bowl, whisk together the chickpea flour, nutritional yeast, baking powder, garlic powder, onion powder, red pepper flakes, and salt. Add the almond milk or soy milk, lemon juice, and ¼ cup water; whisk until smooth. Stir in the spinach.
3. Warm the skillet on low-medium heat. Pour half of the batter into the skillet, and lightly spread out the edges using a silicon spatula so the omelette is 5 to 6 inches in diameter. Cook over low to medium heat until bubbles form across the entire surface of the omelette and the batter begins to firm up, about 5 to 7 minutes.
4. Arrange half of the warmed vegetables on one side of the omelette; cook for another minute. Top the vegetables with half of the chopped avocado (if using), and fold the bare side of the omelette over the filled side.
Press the surface of the omelette gently with the spatula, and cook on low for another minute.