3 tablespoons unsalted butter
3 tablespoons canola oil
1 cup whole-wheat flour
1 1/2 teaspoons baking powder
1/2 teaspoon salt
1 cup reduced-fat milk
1/2 cup sugar
1 teaspoon vanilla extract
3 ripe but firm peaches, (about 1 pound), pitted and sliced into eighths, or 3 1/2 cups frozen
2 cups (1 pint) fresh or frozen blueberries
Preheat oven to 350°F.
Place butter and oil in a 12-inch cast-iron skillet or a 9-by-13-inch baking pan. Heat in the oven until melted and fragrant, 5 to 7 minutes.
Meanwhile, combine flour, baking powder and salt in a large bowl. Add milk, sugar and vanilla; stir to combine.
Add the melted butter mixture to the batter and stir to combine. Pour the batter into the hot pan. Spoon peaches and blueberries evenly over the batter.
Return the pan to the oven and bake until the top of the cobbler is browned and the batter around the fruit is completely set, 50 minutes to 1 hour. Remove to a wire rack to cool for at least 15 minutes. Serve warm.