Preheat the oven to 375°F (190°C). Brush the mushroom caps with olive oil and place gill-side up on a baking sheet. Season with salt and pepper.
In a bowl, mix together chickpeas, spinach, feta, breadcrumbs, and garlic (if using). Season with salt and pepper.
Stuff the mixture into the mushroom caps. Drizzle with more olive oil.
Bake for 20-25 minutes, until the mushrooms are tender and the topping is golden.
Serve warm.