Preheat the oven to 375°F. Toss sweet potato cubes with olive oil, season with salt and pepper, and transfer to a baking sheet. Roast the sweet potato for about 45 minutes, or until soft and slightly caramelized.

Meanwhile, in a large heavy skillet over medium heat, fry bacon until crisp, about 5 minutes. Using a slotted spoon, transfer bacon pieces to a paper towel-lined plate, leaving 2 tablespoons of bacon fat in skillet. Add shallots to the pan and cook under low heat, stirring occasionally, until shallots are softened. Add balsamic vinegar and sugar, and let the vinaigrette reduce by half. Remove from heat and season with salt and pepper. Drizzle warm dressing over salad greens, roasted sweet potatoes, and bacon, and toss gently. Serve immediately.