Step 1Cut the peel off the oranges and grapefruit: With a sharp knife, slice off the very top and bottom of each fruit. Starting at the top and cutting downward along the curve of the fruit, continue to remove the peel from the fruit, cutting away all skin and pith, as well as the outer membrane. Then remove each segment from the fruit by cutting along the membrane between each section; place the segments in a medium bowl.
Step 2Toss the orange and grapefruit segments with the sake. Use a slotted spoon to lift the fruit from the sake and arrange in each of four individual ramekins. Sprinkle 1 tablespoon brown sugar over the top of each ramekin.
Step 3Place the ramekins on a baking sheet and place on the rack closest to the broiler. Broil just until the top is browned, 1 to 1 1/2 minutes, then remove from the oven. Sprinkle the sliced shiso leaves over the top of each ramekin. Serve immediately.