In a large mixing bowl using a hand or stand mixer with the paddle attachment, mix the butter, brown sugar, and granulated sugar on medium-high speed until smooth and creamy.

Add in the egg and vanilla and mix to combine, then mix in the baking soda, cinnamon, kosher salt, and cream of tartar.

Once thats evenly mixed, toss in the quick oats and flour and mix just before its fully combined. Then add in the raisins and use a rubber spatula to fully bring the dough together by hand. It will be very thick and slightly sticky.

Preheat the oven to 350F and line a large baking sheet with parchment paper. Allow the dough to rest at room temperature while the oven heats up.

When ready, use a large cookie scoop (3 tablespoons worth) and place 4-5 cookies onto the pan with a couple inches between. Use your palm to flatten the cookies to about  inch thick.

Bake for 10-12 minutes or until the edges are just barely golden and the center is pale, puffed, and slightly underdone.

When theyre fresh from the oven, use the curve of a fork to nudge in any wonky sides and immediately sprinkle each cookie with a pinch of flaky sea salt. Allow the cookies to rest for about a minute, then transfer to a cooling rack while you continue to bake the rest of the dough.