Sweet Potato Cottage Pie

large sweet potatoes (about 3 lb. total)

 1 cup whole milk

 1/2 cup finely grated Parmigiano-Reggiano

 4 tablespoons unsalted butter, softened 

Kosher salt and Freshly ground black pepper

 4 medium celery stalks, cut into 1/4-inch dice (about 1 1/2 cups)

 2 large carrots, cut into 1/4-inch dice (about 1 1/2 cups)

 2 medium onions, finely chopped (about 3 cups)

Kosher salt and Freshly ground black pepper to taste 

medium cloves garlic, minced

 4 teaspoons ground cumin

 4 teaspoons chopped fresh oregano or 1 teaspoon dried oregano

 2 teaspoons ancho chile powder or other pure chile powder

 1/2 teaspoon ground cinnamon

 3 lbs. ground beef (85% lean)

 1 28-oz. can whole peeled tomatoes

 1 cup coarsely chopped cerignola or other green olives

peppadew peppers, finely sliced

 2/3 cup coarsely chopped dried cranberries