Place amaranth in a small bowl. Stir in 1 cup boiling water. Cover and let stand for at least 1 hour.
Preheat oven to 375 degrees F. Lightly coat a 3-quart rectangular baking dish with nonstick cooking spray. Sprinkle pork with sage and toss to coat. In a large nonstick skillet brown pork in hot oil over medium-high heat. Transfer pork to the prepared baking dish.
In the same skillet cook mushrooms, onion, and garlic to the same skillet over medium heat about 5 minutes or until the onion is tender. Stir in sweet potato and 1 cup water. Bring to boiling. In a small bowl stir together 1/4 cup cold water, the cornstarch, and 1/4 teaspoon of the salt; stir into mixture in skillet. Cook and stir until thickened. Pour over pork in baking dish. (Sweet potatoes will not be done yet.) Bake for 30 minutes. Remove from oven. Increase oven temperature to 450 degrees F.
For biscuit topper, in a large bowl stir together flour, cornmeal, baking powder, thyme, black pepper, and the remaining 1/4 teaspoon salt. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Add soaked amaranth and any liquid remaining in the bowl. Stir until combined.
Using a large spoon, drop biscuit topper into eight mounds (about 1/4 cup each) onto pork stew. Bake for 12 to 15 minutes or until biscuits are browned and a toothpick inserted in the center of a biscuit comes out clean.