Preheat oven to 375°. Place sweet potatoes in a 15x10x1-in. pan; drizzle with olive oil and sprinkle with salt and pepper. Turn to coat. Bake until tender, 30-35 minutes., Meanwhile, in a large skillet, cook bacon over medium-low heat until crisp; drain. Discard all but 4 teaspoons drippings., Cook celery and onion in drippings over medium heat until tender, 6-8 minutes. Stir in garlic; cook 1 minute. Add beef stock and balsamic vinegar; simmer until liquid is reduced by half, 5-8 minutes. Add paprika and, if desired, cumin; cook 1 minute longer., Pour balsamic mixture over sweet potatoes; add bacon. Toss to coat. Top with green onions and, if desired, minced fresh parsley; serve immediately.