Preheat the oven to 350°F (175°C). Lightly grease a 12-cup muffin tin or line with paper liners and set aside.
In a medium bowl, whisk the egg with the maple syrup, vanilla and lemon zest until combined. Stir in the cottage cheese and almond milk; break up the curds with a fork so the mixture is fairly even but still a bit textured.
In a separate large bowl, toss together the oats, baking powder, cinnamon and salt to distribute the leavening and spices evenly.
Pour the wet mixture into the dry ingredients and fold gently with a spatula until no large dry pockets remain. Let the batter rest for 8–10 minutes so the oats absorb liquid and soften; the batter should be thick but scoopable.
Gently fold the mixed berries into the batter, reserving a few to top the cups, then divide the batter among the prepared muffin cups, filling each nearly full. Smooth tops and press a couple of reserved berries into each cup.
Bake on the middle rack for 22–26 minutes, rotating the pan once halfway through, until the tops are set and edges are lightly golden. A toothpick inserted in the center should come out mostly clean.
Allow the cups to cool in the tin for 12–15 minutes so they firm up, then transfer to a wire rack. For best texture, chill for 15–30 minutes before serving — they slice and hold together better when slightly cool.
Serve warm or cold. Store leftovers in an airtight container in the refrigerator for up to 5 days or freeze individually wrapped for up to 2 months.