Heat 2 tablespoons olive oil in a skillet over medium-high heat. Add the potatoes and fry until crispy outside but soft in the middle. Set aside.
In another skillet, heat 1 tablespoon olive oil over medium-high heat. Add the fennel, some salt and the grapefruit juice and saute for a few minutes. Transfer to a bowl and set aside.
In the same skillet the fennel was cooked in, heat another tablespoon olive oil. Add the shallots and saute for a few minutes. Add the shrimp and cook, turning, until light pink, 2 to 3 more minutes. Off the heat, add the pastis. Return the skillet to the burner and carefully tip to flambé. Once the flame burns out, add the cream and cook briefly, stirring. Add the butter and cook until the sauce thickens slightly, another 1 to 2 minutes. Take off the heat.
To serve, place a potato round on a plate. Top with some fennel, followed by a shrimp, a drizzle of sauce, a piece of endive, and half of a grapefruit supreme. Sprinkle with some chives and finish with a small fennel frond.
Use extreme caution when igniting alcohol. Remove the pan from the heat source before adding the alcohol. Pour the alcohol into the pan and carefully ignite with a match or click lighter. Return the pan to the heat and gently swirl to reduce the flames.