Preheat your oven to 350°F (175°C) and line your baking sheets with parchment paper.

In a medium mixing bowl, whisk together the all-purpose flour, baking soda, salt, ground cinnamon, and nutmeg.

In a large mixing bowl, cream together the softened unsalted butter and brown sugar until light and fluffy, about 3-4 minutes.

Beat the egg into the butter and sugar mixture, followed by the vanilla extract and milk.

Gradually add the dry flour mixture to the wet ingredients, stirring until just combined.

Toss the diced rhubarb with a tablespoon of flour and gently fold it into the cookie dough along with the rolled oats if using.

Allow the dough to rest for 10-15 minutes.

Scoop tablespoon-sized rounds of dough onto your prepared baking sheets, spacing them about 2 inches apart.

Bake the cookies for 12-15 minutes, or until the edges are golden and the centers are set but soft.

Allow the cookies to cool on the tray for 5 minutes, then transfer to a wire rack to cool completely.