Preheat oven to 350° F. In a small saucepan, bring potato, carrot, onion, 1 cup water and salt to a boil over medium-high heat.
Lower heat to medium, cover and simmer until potato and carrot are tender, about 10 minutes.
Meanwhile, thaw frozen broccoli in a colander under hot running water, then set aside and drain well.
When potato mixture has finished simmering, pour into a blender and add tofu, yeast, lemon juice and garlic. Blend until very smooth and creamy. Pour into a shallow, greased 1 1/2 to 2 quart casserole. Add broccoli, rice, mushrooms and 3/4 cup water. Stir well and smooth top of casserole. Bake for 40 minutes, until golden and bubbling.
Substitution: Instead of frozen broccoli, you can use 1 lbs fresh broccoli (tops only), chopped and steamed briefly until crisp-tender, then cooled under cold running water and drained thoroughly.
Servings: 4