Let’s break down the process into easy-to-follow steps.

Step 1: Prepare Fruit Mixture. In a large bowl, combine the chopped rhubarb, sliced strawberries, 2/3 cup of the sugar, and the cornstarch. Toss gently to combine.

Step 2: Prepare Baking Dish. Pour the fruit mixture into an ungreased 11×7-inch baking dish.

Step 3: Preheat Oven. Preheat the oven to 350°F (175°C).

Step 4: Cream Butter and Sugar. In a large bowl, cream the softened butter and the remaining sugar until light and fluffy.

Step 5: Add Egg and Flavorings. Add the egg and beat well. Beat in the orange zest and vanilla extract.

Step 6: Combine Dry Ingredients. In a separate bowl, combine flour, baking soda and salt.

Step 7: Alternate Dry and Wet Ingredients. Add the dry ingredients to the creamed mixture alternately with the buttermilk, beating well after each addition. Begin and end with the dry ingredients.

Step 8: Spoon Batter over Fruit. Spoon the batter over the fruit mixture in the baking dish.

Step 9: Bake. Bake in the preheated oven until a toothpick inserted near the center comes out clean, 50-55 minutes.

Step 10: Cool. Cool completely on a wire rack.

Step 11: Serve. If desired, serve with whipped cream and additional orange zest.

These instructions are clear.