To cut the grapefruit into supremes, cut the top and bottom of the grapefruit just so the rind and the white pith are removed and the pink is exposed. Stand the fruit upright, then cut down the sides in sections, following the curve of the fruit so that no rind or pith remain, be careful not to cut too deep. Once the peel and pith are removed, the membranes will be visible. Use a sharp paring knife to cut along the edges of the membrane and remove the center. Repeat until all supremes are removed. (There are tutorials for this on YouTube as well). 

In a large salad bowl, spread the mixed greens into a single layer. Top with the tomatoes, avocado, grapefruit supremes, toasted pecans, and goat cheese. Serve with balsamic dressing.