Position one oven rack in the center and another in the lower third of the oven and preheat the oven to 375°F.
Grease 2 large baking sheets with 2 teaspoons each of the vegetable shortening. Set aside.
In the bowl of an electric mixer, cream the butter and brown sugar together at medium speed until very light and fluffy, about 2 to 3 minutes.
Turn the mixer off before adding the eggs and vanilla extract, then beat until thoroughly combined.
In a medium mixing bowl, combine the oats, flour, baking powder, salt, cinnamon, allspice, and nutmeg and stir to combine.
With the mixer turned off, add the dry ingredients to the mixing bowl. Beat on low speed until dry ingredients are just combined, about 1 minute. Turn the mixer off.
Add the raisins and walnuts, if desired, and mix at low speed until just combined.
Using a 1/4-cup measure, scoop the cookie dough into 24 equal portions and place 12 of the dough balls 2 inches apart on each of the prepared baking sheets. Press the dough down slightly with your hands or the back of a spoon.
Using oven mitts or pot holders, transfer the baking sheets to the oven and bake for 22 to 25 minutes, exchanging positions of baking sheets after 10 minutes.
Using oven mitts or pot holders, remove the baking sheets from the oven and set aside on wire racks for about 10 minutes, or until the cookies are almost completely cooled. Remove the sheets from the racks.
Using a metal spatula, transfer the cookies from the baking sheets to the wire racks to allow them to finish cooling. Be careful-these cookies are fragile while they're warm.
YIELD: 24 extra-large cookies