Preheat the oven to 350°F (180°C). Lightly grease a 9-inch (2.5 L) square baking pan. Line the pan with parchment paper with extra hanging over the sides.

Cut ¼ cup (60 mL) of the marshmallows in half and set aside.

In a high-speed blender, combine the beets, maple syrup, oat milk, olive oil, vanilla, and apple cider vinegar and blend on medium speed for 30 seconds until smooth. Add the almond flour, rolled oats, cocoa powder, flaxseed, baking powder, salt, and cinnamon. Blend on medium speed for 30 seconds, then increase the speed to medium-high and blend until smooth with no lumps, about another 30 seconds. Stir in the walnuts, the remaining ¼ cup (60 mL) whole marshmallows, and the chocolate chips.

Scrape the batter into the prepared baking pan and smooth the top. Scatter the halved marshmallows and the reserved 1 tablespoon (15 mL) chocolate chips evenly over the batter. Bake until the top is dry to the touch and the sides are starting to crack a bit, 42 to 48 minutes. Remove from the oven and let cool in the pan for 10 minutes, then carefully lift the brownies onto a rack using the parchment paper overhang and let cool completely. Store the brownies, loosely covered, on the counter for up to 4 days or tightly wrapped in the freezer for up to 1 month.