Preheat oven to 375 degrees F. Combine the peaches, sugar, Grand Marnier, and slurry together in a sauce pot. Bring up to a boil and reduce to a simmer. Reduce to a syrup consistency, about 3 to 4 minutes.
For the batter: In a standing electric mixer, whip the sugar and eggs together until light in color and ribbon like texture, about 3 minutes. Mix the milk and butter together and incorporate into the egg mixture. Fold the flour into the egg mixture. In a 13 by 9-inch baking pan, cover the bottom of the pan with the peach mixture. Spoon the batter over the peaches, spreading evenly. Bake for 30 to 35 minutes for until the batter has set and is slightly golden brown. Spoon the clafouti into a bowl and top with the peach ice cream. Garnish with fresh mint and cinnamon.