Preheat your oven to 375°F (190°C). This ensures your muffins bake evenly and rise nicely.
Grease a muffin tin with a little oil or line it with muffin liners to prevent sticking.
In a large mixing bowl, combine the egg, mashed bananas, oat milk, and vanilla extract. Whisk together until well-combined.
In another bowl, mix the rolled oats, maple syrup, and sea salt. Stir to combine these dry ingredients.
Pour the dry mixture into the bowl with the wet ingredients. Stir until just combined, being careful not to overmix.
Gently fold in the mixed berries and chopped Honeycrisp apple until evenly distributed throughout the batter.
Spoon the batter into each muffin cup, filling them about 3/4 full to allow for rising.
Bake in the preheated oven for about 20-25 minutes or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the tin for a few minutes before transferring them to a wire rack. Serve warm and enjoy!