1 (12-ounce) package jumbo shells pasta
2 tablespoons olive oil
1/2 pound thick-cut pancetta, cut into 3/4-inch cubes
2 pounds frozen spinach, thawed and drained
1 (15-ounce) container whole milk ricotta
1 cup grated asiago cheese
1/2 teaspoon freshly ground black pepper
1/4 teaspoon freshly grated nutmeg
1 tablespoon butter
1 garlic clove, minced
1 cup cream
2 cups grated asiago cheese, plus 1/4 cup
1/4 cup chopped fresh parsley
1/4 teaspoon freshly ground black pepper