Preheat your oven to 350°F and line a 12-cup muffin tin with paper liners or grease it well. While the oven heats, gather all your ingredients and prepare your workspace. Slice the rhubarb into 1/2-inch pieces and set aside—this size ensures even baking and prevents large pieces from remaining tough. I like to have the rhubarb prepped and ready to fold in at the last moment so it stays as separate pieces rather than breaking down during mixing.

In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and ground nutmeg. Whisking rather than just stirring helps distribute the leavening agents evenly throughout the flour, which ensures your muffins rise consistently and have a tender crumb. Set this mixture aside.

In a large bowl, mash the bananas until smooth and completely free of lumps—this ensures even moisture distribution throughout the batter. Add the room-temperature eggs and stir vigorously for about 30 seconds to begin creating an emulsion. Then add the Greek yogurt, vegetable oil, vanilla, and sugar, mixing until everything is well combined and smooth. Room-temperature eggs blend much more smoothly into the batter and help create a unified, tender crumb rather than resulting in dense pockets.

Pour the wet ingredient mixture from Step 3 into the bowl with the dry ingredients from Step 2. Stir gently with a spatula just until the dry ingredients are moistened—do not overmix, as this can toughen the muffins by overdeveloping gluten. The batter should look slightly lumpy and rough. Gently fold in the prepared rhubarb pieces in the final fold, being careful not to crush them so they stay as distinct pieces in the finished muffin.

Divide the batter evenly among the 12 muffin cups, filling each about two-thirds full. This gives the muffins room to rise without spilling over. Place the tin in your preheated 350°F oven and bake for 18-20 minutes, until a toothpick inserted into the center of a muffin comes out with just a few moist crumbs clinging to it—don't bake until completely clean, as that can result in dry muffins.

Remove the muffin tin from the oven and let the muffins rest in the tin for about 5 minutes. This brief cooling period allows the structure to set just enough so they'll come out cleanly without falling apart. Turn the muffins out onto a wire rack to cool completely, which prevents condensation from making them soggy on the bottom.