Position racks in the upper and lower thirds of the oven and preheat to 425 degrees F. Combine the mushrooms, bell peppers, zucchini, walnuts, half of the garlic, 2 tablespoons olive oil, 1/2 teaspoon salt, and pepper to taste in a bowl; toss, then divide between 2 rimmed baking sheets. Roast, stirring once or twice, until the vegetables are tender and browned in spots, about 25 minutes.
Meanwhile, combine the beans, the remaining garlic, the lemon juice, the remaining 1 tablespoon olive oil, 1/2 teaspoon salt and 1/3 cup water in a blender; puree until smooth. Add the parsley and pulse until just combined.
Heat the brown rice as the label directs. Divide among plates and top with the roasted vegetables, bean puree and sprouts.