Preheat the oven to 425 degrees F. Combine the rhubarb, brown sugar, salt, cinnamon, cardamom and lemon zest in a small bowl. Set aside. Combine the oats and buttermilk in a large bowl. Set aside. Both mixtures need to sit for approximately 15 minutes while everything else is prepared.
Prepare the streusel in a small bowl by combining the brown sugar, spelt flour, butter and lemon zest. Rub and mix with your fingers until it resembles course crumbs.
Mix together the spelt flour, baking powder and baking soda in a small bowl. In a glass measuring cup, whisk together the oil, eggs and vanilla extract.
Add the oil mixture to the oats and buttermilk mixture and stir to combine. Gently fold in the flour mixture until almost mixed and then add the rhubarb mixture. Fold until completely incorporated.
Line a muffin pan with muffin cups or grease the muffin pan with butter. Fill each muffin cup really full so that each one is heaped. Top with streusel.
Bake at 425 degrees F for 10 minutes. Lower the temperature to 375 degrees F for another 12 - 15 minutes or until a toothpick poked in the centre of a muffin comes out clean.
Remove from the oven and let the muffins cool for at least 20 minutes in the muffin pan before removing to finish cooling on a wire rack.