In a small bowl, pour boiling hot water over the raisins (or sultanas) until completely covered. Set aside for about 5-10 minutes, until the raisins have plumped up and re-hydrated.Drain the water, making sure to squeeze out any excess water from the raisins. Set aside until needed.

In a large bowl, whisk together the softened butter, light brown sugar, granulated sugar and orange zest until pale and somewhat fluffy (but no need to get it super fluffy, like you might when making a sponge cake or buttercream). You can do this by hand using a balloon whisk, with a stand mixer fitted with the paddle attachment or with a hand mixer fitted with the double beaters.Tip: You can rub the orange zest into the granulated sugar with your fingertips before adding them to the rest of the ingredients, as that helps to release the essential oils from the zest and makes the cookies even more fragrant.

Add the egg and vanilla, and whisk until combined.

In a separate bowl, whisk together the gluten free flour blend, xanthan gum, baking powder, baking soda, salt and cinnamon, and add them to the butter-sugar mixture.

Use a rubber spatula or a wooden spoon to mix it all together until you get a smooth dough. Note that the dough will be VERY soft and sticky at this stage.

Add the oats, re-hydrated raisins and chopped walnuts. Mix it all together until combined and the add-ins are evenly distributed within the cookie dough.

Chill the cookie dough in the fridge for about 30-45 minutes.Tip: If you like your oatmeal cookies thinner and crisper, you can skip this chilling step. If you prefer them thicker and chewier, you can chill them for up to 1 hour. See the blog post for more details and photos.

While the cookie dough is chilling, adjust the oven rack to the middle position, pre-heat the oven to 350ºF (180ºC) and line 2-3 large baking sheets with baking/parchment paper.Tip: You will bake the cookies in several batches, so you can line 2-3 baking sheets if you have them on hand. Otherwise, just re-use the same baking sheet, but make sure to cool it completely before you place the next batch of unbaked cookies onto it.

Once chilled, use a "2 tablespoon" ice cream or cookie scoop to scoop individual portions (balls) of cookie dough onto the lined baking sheets. You should get 18 cookies in total. Make sure to leave plenty of space around the cookies on the baking sheet, as they will spread out during baking – I usually fit about 6-8 cookies per baking sheet.

Bake the oatmeal cookies, one baking sheet at a time, at 350ºF (180ºC) for 10-12 minutes or until they are light golden brown around the edges. The centres of the cookies will look under-baked (their tops will be very soft and shiny) – that's okay. They should and have to look underbaked if you want to get the perfect soft and chewy texture.Tip: If your cookies aren’t perfectly round, you can correct their shape while they’re still very hot immediately out of the oven. Take a round cookie cutter that’s larger than the cookie diameter, and use it to nudge or scoot the cookies into a more round shape.

Allow the cookies to cool on the baking sheet for at least 5-10 minutes before transferring them onto a wire cooling rack to cool completely.Tip: As the cookies cool, they will set and lose their under-baked appearance. They will end up perfectly soft and chewy.

These gluten free oatmeal cookies keep well stored in an airtight container at room temperature for about 1 week.

If you’re making these cookies exclusively with the intention to freeze them for baking later, you can make them with about 20-30g (3-4 tablespoons) of gluten free flour blend less than is required in the recipe below.

Then, chill the cookie dough in the fridge until it’s firm enough to scoop, and freeze the scooped-out balls of cookie dough (in an airtight container or a ziplock bag) until you want to bake them.Tip: By adding less flour, you’re ensuring that the cookies will spread out properly, even after you’ve kept them in the freezer for a few weeks. Over time, the oats absorb more and more of the moisture in the cookie dough – that's why you need to use less flour if you'll be keeping the unbaked cookies for a longer period of time.

Bake them directly from frozen, you might need to bake them for a few minutes longer to account for the extra thawing time in the oven.