This recipe from "Cake Mix Magic" uses lemon cake mix for the base. Several Duncan Hines cake mixes, including the lemon, are MSPI-friendly. The only change I made was substituting dairy-free margarine for butter. Also, serve the whipped cream on the side for those who can eat it. If berries aren't in season and are super expensive, try using frozen berries that have been thawed.
Bumbleberry Lemon Upside-Down Cake
1/3 cup dairy-free margarine (butter)
1 cup packed brown sugar
1 1/2 cups fresh raspberries
1 1/2 cups fresh blueberries
1 cup fresh strawberries
1 pkg. lemon cake mix
Whipped cream (optional)
In a 9- by 13-inch cake pan, melt margarine. Sprinkle brown sugar evenly on top. Scatter fruit over sugar.
Prepare cake mix according to package directions. Pour batter evenly over fruit. Bake 45 to 50 minutes or until a toothpick inserted in center of cake comes out clean. Let stand 5 minutes, then turn upside down onto a large platter or a cookie sheet. Serve warm with a dollop of whipped cream, if desired.