Portabellos with Herbed Stuffing and Sweet Potatoes

medium sweet potato (about 12 oz.), peeled and cut into ½-inch chunks

Olive oil

Salt and pepper

portabello mushrooms

small onion, diced small

stalk celery, diced small

fresh thyme leaves

minced fresh sage

cubed or torn whole grain bread (crusts removed)

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vegetable stock

large egg, lightly beaten