Mix together yogurt, 2 Tbsp butter, oil and egg. In a separate bowl mix flour, ¾ C brown sugar, soda and salt. Combine wet ingredients into dry. Add rhubarb. Fill muffin tins (lined or greased) with 2/3 cup of mixture each. Mix ¼ C brown sugar, cinnamon, nutmeg, and 1 tsp butter. Spoon over muffins and press down. Bake @ 350 for 25 minutes. Cool 15 minutes then remove from muffin tin to complete cooling.