Working with 1 orange at a time, cut a thin slice off the top and bottom, exposing the flesh. Stand the orange upright and, using a sharp knife, thickly cut off the peel, following the contour of the fruit and removing all of the white pith and membrane. Holding the orange over a small bowl, carefully cut along both sides of each section to free it from the membrane. As you work discard any seeds and let the sections and any juice fall into the bowl. Repeat with the other orange and the grapefruit.
In a separate bowl, whisk together the orange juice, olive oil and vinegar. Pour the mixture over the fruit segments and toss gently to coat evenly. To serve, divide the spring greens among individual plates. Top each with the fruit and dressing mixture and sprinkle each with 1/2 tablespoon pine nuts. Garnish with chopped mint (optional). Serve immediately.