Grease and flour a 8-inch round spring form pan with removable bottom.
Preheat oven to 350F.
Add the peach slices and blackberries into a small bowl and squeeze the lemon juice over it; set aside.
In a medium sized bowl, sift together the gluten-free flour (I use Bob's Red Mill 1-1 or King Arthur's Flour), baking powder, salt and cinnamon.
Toss in the lemon zest and with your hands, mix it into the flour mixture to release the lemon essence.
In a large bowl with an electric mixer (or by hand), beat the sugar and eggs until creamy (about 5 minutes). Add in the oil and the yogurt. Mix well.
Add flour mixture into the yogurt mixture a half cup at a time.
Scrape down the sides of the bowl with spatula and mix well after every addition.
Then pour the batter into prepared pan. Arrange the peach slices blackberries over the batter.
Sprinkle the turbinado sugar and thin pats of margarine (or butter) on top of the cake and bake for about 40-45 minutes, or until skewer inserted in the center comes out clean. Use a wooden skewer to check if it’s ready. Best to check cake at about 40 minutes. Every oven is different. If cake is browning, cover with aluminum foil for remainder of baking time.
Let cool for about 20 minutes before removing from cake pan.
Best served warm. Store in the refrigerator for up to 4 days.