Put the potatoes in a small, heavily salted pot of cold water with the smashed garlic. Bring the water to a boil, then cook until the potatoes are tender, 4 to 5 minutes. Drain and cool on a quarter sheet tray.
Using kitchen shears and starting below the head, slide a blade along the back of a shrimp until you reach the tail. Peel the skin off, being careful to keep the head and tail intact. Devein if necessary and very briefly rinse. Repeat with the remaining shrimp.
Wrap each piece of shrimp with a slice of prosciutto and secure with a toothpick. Thinly slice the remaining prosciutto and set aside.
Heavily coat a pan in olive oil and place over medium-high heat. Working in batches, saute the shrimp until the prosciutto is crisp and the shrimp has turned pink, about 2 minutes per side. Remove the shrimp from the pan and let cool, then remove the toothpicks.
Using the same pan and additional olive oil if necessary, fry the extra prosciutto slices until crisp, about 2 minutes. Turn down the heat to medium and add the potatoes. Cook for 2 minutes, then add the red onion. Cook until the onions are soft, about 4 minutes, then add the red wine vinegar and lightly mix. Turn the heat to low and add the fennel, reserved juice from the grapefruit and the red pepper flakes. Reduce for an additional minute or two. Add the radicchio and arugula and gently toss to coat. Season to taste.
Plate a large spoonful of warm salad and top with 3 pieces of shrimp. Garnish with a couple grapefruit supremes and fennel fronds. Repeat to make one more serving