Preheat your oven to 200°C (about 390°F). Cut the fresh rhubarb into 1-inch lengths and place on a baking tray with 1/3 cup sugar. Cover tightly with foil and roast for 20 minutes until the rhubarb begins to soften and release its juices. Remove the foil and roast uncovered for another 5 minutes to concentrate the flavor slightly. Once done, drain the rhubarb in a sieve, reserving any liquid that collects—this prevents the muffins from becoming too wet while keeping the filling flavorful.

While the rhubarb roasts, prepare the crumble topping by combining the margarine, self-raising flour, brown sugar, and ground cinnamon in a bowl. Use your fingertips to work the margarine into the flour mixture until it resembles coarse breadcrumbs with pea-sized pieces of margarine still visible. I use Blue Bonnet margarine because it creates a more textured, chunky crumble that bakes up beautifully—avoid fully incorporating the margarine into flour dust. Set the crumble mixture aside until you're ready to top the muffins.

In a large bowl, whisk together the all-purpose flour, baking powder, and 3/4 cup sugar until evenly combined. Make a well in the center of the dry mixture—this helps you combine wet and dry ingredients more efficiently without overmixing the batter.

In a separate jug, whisk together the room temperature egg, sunflower oil, and Greek yogurt until smooth and well blended. I always use Fage plain Greek yogurt because it creates a thicker, more luxurious batter that results in moist muffins. Pour the wet ingredient mixture into the well of the dry ingredients and fold together gently with a spatula just until the flour is incorporated—be careful not to overmix, as this develops gluten and can result in tough, dense muffins. A few small lumps in the batter are perfectly fine.

Line a 12-cup muffin tin with paper cases or grease lightly. Divide the batter from Step 4 evenly among the 12 muffin cases, filling each about two-thirds full. Top each muffin with approximately one tablespoon of the drained rhubarb from Step 1, then sprinkle generously with the crumble mixture from Step 2, distributing it evenly across all twelve muffins. Bake at 200°C for about 20 minutes until the tops are golden brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs. Allow the muffins to cool in the tin for 10 minutes before turning out to cool further on a wire rack.