Roasted Sweet-Potato Spears with Bacon Vinaigrette

4 Lbs medium sweet potatoes (about 7)

½ lb sliced bacon, cut crosswise into ½-inch strips

1 teaspoon salt

½ teaspoon pepper

¼ cup extra-virgin olive oil

3 scallions, thinly sliced, reserving sliced greens separately

2 tablespoons sherry vinegar

1 tablespoon water

Preparation;

Preheat oven to 450 degrees.  Peel sweet potatoes then cut each lengthwise into 6 spears.  Arrange in 1 layer in a large shallow baking pan.

Cook bacon in a heavy skillet over moderate high heat, stirring, until browned and crisp, 10-15 minutes.  Transfer bacon with a slotted spoon to paper towels to drain.  Pour bacon fat through a fine-mesh sieve over the potato spears and toss with two spatulas to coat.  Sprinkle spears with ½ of the salt and pepper and roast uncovered turn every 15 minutes until potatoes are tender and edges are browned, 45 minutes to 1 hour.  Transfer to a serving dish.  Return bacon to cleaned skillet, and then add oil and heat over moderate high heat until hot but not smoking.    Stir in white parts of the scallions and remove from heat.  Stir in vinegar, water, remaining salt and pepper, and pour over the potato spears.  Sprinkle with scallion greens and serve warm.