Preheat oven to 400°F (200°C). Heat 1 tbsp olive oil in an 8-inch oven-safe skillet over medium heat. Sauté onion, bell pepper, and mushrooms until softened, about 5 minutes.

While vegetables cook, whisk chickpea flour, water, nutritional yeast, baking powder, salt, and pepper until smooth. Let batter rest 5 minutes.

Pour batter over vegetables in the skillet. Top with spinach and drizzle remaining 1 tbsp olive oil. Bake 22 to 25 minutes, until set in the center.

Cool in the pan for 10 minutes, then loosen edges with a knife, slide out, and slice into 4 wedges. Serve warm or at room temperature.