Preheat the oven to 375°F (190°C). Line 2 baking sheets with parchment paper or silicone baking mats.
Combine 1 tablespoon of flaxseed meal with 3 tablespoons of room temperature water in a small bowl. Let it sit for 10–15 minutes to thicken into a gel like consistency.
Sift the flour, ground cinnamon, baking powder, baking soda, and salt (if using) into a medium bowl. Set aside.
Use an electric mixer to cream the vegan butter and brown sugar for about 2 minutes on medium speed. You can also use a stand mixer. The mixture will become light and fluffy. Do not over whip.
Add the vanilla extract, plant milk, and flax egg to the creamed butter mixture, and mix on low until just blended.
Gradually add the flour mixture a little at a time to the wet butter mixture, mixing on low just until combined. Then fold in the oats, raisins, and walnuts with a rubber spatula by hand or on low with your mixer. Do not overmix.
Refrigerate the dough for 30 minutes. This step is optional but I find it produces the best results.
Scoop 1 heaping tablespoon of dough, form it into a ball, and place it on the prepared baking sheets. Repeat the process leaving 2 inches of space between each cookie ball. Then gently flatten each cookie a littlw with your hand.
Bake the cookies for 12-14 minutes, until the edges and bottoms are lightly browned. Rotate the pans halfway through the baking time if you're baking more than one sheet at a time.
Let cookies sit on baking sheets for about 5 minutes, then transfer to a wire cooling rack.
Store in an airtight container for up to 5 days at room temperature or in the freezer for up to 3 months.