Cut the rhubarb stalks crosswise into ¼-inch slices.
In a medium bowl, combine the rhubarb with 2 tablespoons brown sugar, ½ teaspoon cinnamon, and ¼ teaspoon cardamom. Set aside.
In a large mixing bowl, whisk together the eggs, canola oil, oat milk, and vanilla.
In a separate bowl, combine the oat flour, oats, flax, baking powder, baking soda, salt, and lemon zest with the remaining brown sugar, cinnamon, and cardamom.
Add the rhubarb to the flour mixture, tossing to coat.
Add the rhubarb and dry ingredients to the egg mixture, stirring until the mixture is just combined and no dry spots remain.
Use a large ice cream scoop or spoon to fill the muffin papers two-thirds full.
To make the streusel, combine all of the ingredients in a small bowl and mix well.
Divide the streusel evenly between the muffin batter tops.
Bake the muffins for 25–30 minutes, until a tester inserted in the center comes out clean.
Cool the muffins for 15 minutes in the pan, then remove and finish cooling on a rack.