Preheat the oven to 350F and line two cookie sheets with parchment paper or a silicone liner.

Add the butter to a stand mixer that has been fitted with the paddle attachment. Whip the butter on medium-high speed until ultra fluffy, about 3 minutes. Stop and scrape down the sides of the mixer bowl one to two times during the process.

Add the brown sugar and white sugar to the stand mixer, and cream the sugars and butter together until light and fluffy, about 3-4 minutes. Scrape down the edges of the bowl as needed.

Add the egg and the egg yolk to the butter and sugar mixture, as well as the vanilla extract. Beat briefly on medium speed until the egg is fully incorporated.

Add the sourdough discard and mix briefly again, just until the mixture is uniform.

In a separate large bowl, add the flour, salt, baking soda, cinnamon, and nutmeg. Whisk to combine.

Add the dry ingredient mixture to the wet ingredients and mix on medium speed just until combined.

Add in the walnuts (if using) and raisins. Mix again just until the walnuts and raisins are evenly distributed.

Scrape down the edges of the bowl and add the rolled oats. Mix until just combined, and be careful not to overwork the dough. You are looking for a dough that easily sticks together when formed into a ball, although it won't be perfectly smooth.

Using a cookie scoop or tablespoon, portion the dough into 1 ½" rough mounds.

Place on the prepared cookie sheets, 2" apart, and at about 12 cookies per half size sheet.

Bake the first cookie sheet in the oven for 10-12 minutes, or until just the edges are beginning to brown ever so lightly. They will look slightly undone and soft in the middle, and this is okay!

Remove the cookies from the oven, and bake the next prepared sheet of cookies for 10-12 minutes.

Allow your cookies to cool on the baking sheet for 2-5 minutes before transferring to a wire cooling rack to cool completely.

Once the baking sheet has cooled, prepare the next batch of cookies for the oven. Rotate the sheets back and forth until all of your cookies are baked.