Place an oven rack in the center of the oven. Preheat the oven to 350 degrees F.
Place the thawed peaches and cherries on a paper towel-lined plate to remove any excess juices. Set aside.
In a medium bowl, whisk the eggs, milk, sugar, thyme, and salt until smooth. Stir in the fruit.
Heat the oil in a 10-inch, oven-safe, nonstick skillet (without a rubber or plastic handle) over medium heat. Pour in the egg mixture and, using a wooden spoon, carefully distribute the fruit evenly in the pan. Sprinkle the cheese over the top and cook without stirring for 5 to 6 minutes, or until the edges start to set. Transfer the skillet to the oven and bake until the frittata is slightly puffed and the egg mixture has set, 25 to 30 minutes. Cool for 5 minutes. Using a silicone or rubber spatula, loosen the edge of the frittata and slide onto a platter. Cut into wedges and serve with maple syrup.