Preheat oven to 350°F. Grease and line the bottom of a 9x5 inch loaf with parchment paper.In a small bowl, mix together the flour, oats, and brown sugar. Crumble in the butter with your fingers combining with the flour until pea-sized lumps are left. Set aside in the fridge.In a medium bowl, sift the flours, baking powder and salt together. Set asideIn another bowl, whisk the sugar and lemon zest. Mix in eggs, lemon juice, lemon zest and vanilla extract together. Add the melted butter, coconut oil and Greek yogurt whisking until fully incorporated.Slowly add the flour mixture to the batter in three additions, whisking gently after each addition until flour is just combined.Pour batter into prepared loaf pan. Evenly sprinkle with oat topping.Bake in preheated oven for 15 minutes before turning down the heat to 325°F. Continue baking for 30-35 more minutes, or until tops are a golden brown color and a toothpick comes out clean.Allow cake to cool completely on wire rack.Whisk together powdered sugar and ½ of the lemon juice. Add more juice slowly as needed since you want it on the thick side. Pour over top of cooled cake.