Preheat oven to 350°. Grease and flour a 10-in. fluted tube pan. Finely grate enough zest from lemons to measure 2 tablespoons. Cut lemons crosswise in half; squeeze juice from lemons to measure 1/4 cup. Save remaining juice for another use.
In a large bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Add 1 egg at a time, beating well after each addition. Beat in lemon juice and zest. In another bowl, whisk flour, baking powder, baking soda and salt; add to creamed mixture alternately with buttermilk, beating well after each addition.
Transfer batter to prepared pan. Bake until a toothpick inserted near the center comes out clean, 50-60 minutes. Cool in pan 10 minutes before removing to a wire rack.
Meanwhile, for topping, combine sugar and rhubarb in a small saucepan. Bring to a boil; reduce heat. Simmer until rhubarb is almost tender, 8-10 minutes. Add halved strawberries; cook until strawberries and rhubarb are softened. Serve with cake. If desired, dust with confectioners' sugar.
Test Kitchen Tip
Try replacing the strawberries with chopped apples for a uniquely different flavor twist.