Make the Flax Egg: In a small bowl, whisk together 1 tbsp ground flaxseed and 3 tbsp water. Set aside for 5–10 minutes until it thickens into a gel. This is your egg replacement.FYI if you do not want to use a flax egg, you can use a regular egg if that is your dietary preference. Make the streusel: In a separate small bowl, combine all-purpose flour, brown sugar, and ground cinnamon. Drizzle in melted butter and use a fork to work it together until crumbly clumps form. Place in the fridge while you prepare the batter.
Prep the rhubarb: Chop fresh rhubarb stalks into ½-inch pieces. In a medium bowl, toss 1½ cups rhubarb with 1 tbsp all-purpose flour until lightly coated. This prevents the rhubarb from sinking to the bottom during baking.
Preheat oven: Preheat your oven to 375°F (190°C). Line a standard 12-cup muffin pan with paper liners or grease each cup well (I personally prefer the greasing method so I don't have to risk them sticking to those paper liners - just grease thoroughly!)
Mix the dry ingredients: Mix together the dry ingredients in a large bowl: flour, brown sugar, baking soda, baking powder, cinnamon, nutmeg and salt.
Mix the wet ingredients: In a medium bowl, whisk together soy milk, melted vegan butter, vanilla extract, and your flax egg until smooth with no streaks of butter.
Pour the wet ingredients into the large bowl of dry ingredients. Use a spatula to fold gently — about 10 to 12 strokes. Stop while a few flour streaks remain.
Fold in the rhubarb: Add the flour-coated rhubarb to the muffin batter. Add walnuts too if using. Fold in with the spatula just until evenly distributed. Do not overmix.
Fill muffin pan: Divide the batter evenly among the muffin tins, filling each cup about ¾ full. Remove the streusel from the fridge and sprinkle generously over each muffin.
Bake for 22–25 minutes without opening the oven door early. At 22 minutes, insert a toothpick into the center of a middle muffin — it should come out clean or with a few moist crumbs.
Let the muffins rest in the pan for 5 minutes, then transfer to a wire rack. Best served slightly warm with a little butter or jam. :)