In a large bowl beat together the sugar cookie mix, flour, egg, butter, cream cheese, lemon zest and juice until a stiff dough is formed. Remove the dough from the bowl and divide it into 2 balls. Wrap them in plastic wrap and refrigerate for at least 1 hour. (At this point the dough can be kept in refrigerator for up to 2 days or frozen for 2 months.)
Preheat the oven to 375 degrees F.
On a lightly floured surface with a floured rolling pin, roll the dough to 1/3-inch thickness. Using the square and round cookies cutters, dipped in flour, cut out an equal number of each shape and transfer them to an ungreased cookie sheet. Bake until the edges are golden brown, about 8 to 10 minutes. Remove the cookies from oven and cool slightly before transferring to a wire rack to cool completely. Repeat with remaining dough, making sure to cool down the baking sheets between batches.
Cut the fondant into 4equal pieces. Lightly dust a clean, dry, flat surface with cornstarch. Working in bathes, roll out each piece to 1/4-inch thickness. Using the round and square cutters cut out enough fondant to equal the number of round and square cookies. Using small flower cutters cut out flower shapes from each piece of fondant. Dampen 1 side of each flower shape with a pastry brush and attach it to the cooled cookies.