Step 1 In a small saucepan, melt 2 tablespoons of butter over medium heat. Add the onions and stir to coat with the butter. Cover the pan and cook for about 5 to 7 minutes, stirring occasionally, until the onions are translucent.
Step 2 Using a rasp grater, grate the garlic directly onto the onions (or mince the garlic and add it). Add the cream and bring to a simmer. Cover, reduce the heat to low and cook gently about 6 minutes, or until the onions are very soft.
Step 3Remove the onions from the heat and stir in the remaining 14 tablespoons butter. Return to the heat and stir until the butter has melted and the mixture is combined. Remove from the heat.
Step 4Place the raw livers in a blender and add the onion mixture, salt and pepper. Blend until thoroughly combined, scraping down the sides of the blender as necessary. Transfer to a large measuring cup or a bowl that has a spout. Heat the oven to 300 degrees. Bring a pot of water to a boil.
Step 5Place four small ovenproof bowls or crocks with about 1-cup capacity into a large deep baking dish, arranging them so the bowls do not touch each other or the sides of the pan. Using a small strainer, strain the mousse into the bowls, dividing it evenly.
Step 6Cover each bowl with a piece of foil and place the baking dish on the oven rack. Ladle or pour into the baking dish enough boiling water to reach halfway up the sides of the bowls. Cook for 30 minutes, or until the mousse is set. Remove from the oven and let cool to room temperature. Refrigerate the mousse for a few hours to chill. Once the mousse is cold and set, prepare the gelee.
Step 7Place the cucumber in a food processor and process until liquefied. Strain through a fine-mesh strainer into a measuring cup. You should have one-half cup cucumber water. In a small microwave-safe bowl, combine the gelatin and one-fourth cup of the cucumber water. Add the lemon juice, sugar, salt and Tabasco to the remaining cucumber water. Place the small bowl in the microwave and heat just to melt the gelatin; do not allow to boil.
Step 8Stir the gelatin mixture back into the seasoned cucumber water. Stir in 2 tablespoons of the parsley. Add some or all of the remaining parsley a little at a time until the mixture is dense with parsley but the green of the cucumber water is still visible.
Step 9Remove the faux gras from the refrigerator and spoon about 2 tablespoons of the gelee on top of each container. Refrigerate until the gelee is set, about 1 hour. (The assembled mousse can be refrigerated for up to 3 days before serving.) Let stand at room temperature for 30 minutes before serving. Serve with crusty bread.