Heat oven to 450°F. Line a baking sheet with foil and lightly coat with nonstick cooking spray.
Scrape gills out of mushrooms using a spoon, then arrange portobellos on prepared sheet, upside down. Roast mushrooms until just beginning to soften, about 10 minutes. Remove from oven and let cool on pan.
Toast grains in a saucepan until fragrant. Add broth and bay leaf, bring to a boil, then reduce to low, cover and simmer 10 minutes. Stir in currants, cover and simmer 2 minutes more. Remove pan from heat and let stand, covered, for 10 minutes. Fluff cooked grain mixture with a fork, discard bay leaf, and season with salt and pepper.
Heat oil in a sauté pan over medium. Add onions and cook until softened, 3 minutes. Add chicken and cook 5–6 minutes; season with salt and pepper. Add carrots, garlic, curry, cumin, and paprika and cook 1 minute. Off heat, stir in cooked grain mixture, chickpeas, olives, and parsley; season with salt and pepper.
Divide mixture between mushroom caps, arrange on baking sheet and bake until mushrooms are tender and filling is heated through, 15–20 minutes. Sprinkle with additional parsley and serve.