To make the omelet batter, mix together the chickpea flour, nutritional yeast, onion powder, garlic powder, turmeric, baking powder, and black salt. Add the water and stir until a batter is formed. It should be pourable and not too thick so add a little more water, if necessary. Let the batter sit while you make the filling.
In a non-stick pan, cook the onion and salt until the onions have softened. Add the mushrooms and cook until they are softened and lightly browned, about 8 minutes. Add the cherry tomatoes and garlic and cook until the garlic is aromatic and the spinach is wilted and much of the water has evaporated.
Transfer vegetables to a bowl. Wipe out the pan.
Heat the pan over medium heat and add a little oil. (Sometimes non-stick pans stick!) Scoop up about 1/2 cup of the batter with a measuring cup. Pour it into the pan and quickly swirl to cover the bottom of the pan with batter. Cook until bubbles form and the batter cooks through until it springs back when touched. Loosen the omelet to ensure it's not stuck.
Spoon vegetables onto half of the omelet and fold one side over. Transfer to a plate and add hot sauce or ketchup then dig in!