Preheat oven to 375°F. Prepare muffin tins by spraying with non-stick pan spray or greasing lightly with vegetable oil.
Mix the rhubarb with the brown sugar and 1 tsp. ginger in a medium bowl and set aside.
In a large bowl whisk together eggs, oil, Greek yogurt, buttermilk, and vanilla.
Shake jar of muffin mix till ingredients are evenly distributed.(**See note below.) Then pour contents of jar into egg mixture. Add ground flax seed, if using. Stir together just until dry ingredients are moistened.
Gently fold sliced rhubarb into batter. In a small bowl, mix together the turbinado sugar, 1/2 tsp. ginger, and sliced almonds.
Fill muffin cups about 2/3 full with prepared batter. Sprinkle the sugar, ginger, and almond topping evenly across all muffins.
Bake at 375°F for 15 minutes or until done. Remove from oven and from muffin cups and cool the muffins on a wire rack.
Makes about 20 muffins.