Preheat oven to 350°F. Line a 12-cup muffin pan.
In a large bowl, whisk the flour, oats, flaxseed, brewer's yeast, cinnamon, baking powder, baking soda, and salt.
In another bowl, whisk the eggs, Greek yogurt, maple syrup, oil, and vanilla. Add the wet to the dry and mix just until combined, then fold in the apples.
Divide the batter among the 12 muffin cups. Mix the streusel ingredients and sprinkle over each muffin.
Bake 18 to 22 minutes, until a toothpick inserted in the center comes out clean.
Cool in the pan 5 minutes, then move to a wire rack.