2 Granny Smith or red delicious apples, cored and diced
1/2 tsp xanthan gum
Heat oven to 350° F.Grease (with olive oil or butter) and dust with rice flour a non-stick Bundt pan.
For the streusel, in a medium size bowl, stir brown sugar, rice flour and cinnamon with a fork or rub together with fingertips until crumbly and butter is completely absorbed.Stir in walnuts.
For the cake, mix flours, baking soda, baking powder, and xanthan gum in a medium bowl.Beat butter and honey in a large bowl with an electric mixer until fluffy (takes about 2 minutes).Beat in eggs one at a time, beating well after each.Beat in vanilla and yogurt with mixer on low speed.Beat in flour mixture just until blended, scraping down bowl as necessary.
Spoon 3 cups of batter into pan, spreading evenly.Sprinkle with 1/4 of streusel, apples, and then a half cup of streusel.Spoon on remaining batter and spread evenly.Sprinkle with remaining streusel, pressing down lightly so it sticks to batter.
Bake 50-60 minutes or until a pick inserted in cake comes out clean.Cool in pan on wire rack for 15 minutes.Place cookie sheet over pan, and carefully invert both.Remove pan and cool completely to serve inverted onto serving plate.