In a large bowl, combine the first six ingredients. In another bowl, whisk the egg, buttermilk, oil and vanilla. Stir into dry ingredients just until moistened. Fold in strawberries and rhubarb. Fill greased or paper-lined muffin cups two-thirds full. In a small bowl, combine pecans, brown sugar and cinnamon. Cut in butter until mixture resembles coarse crumbs. Sprinkle over batter. Bake at 400 degrees for 20-25 minutes or until a toothpick comes out clean. Cool for 5 minutes before removing from pans to wire racks. Makes 1 1/2 dozen.