Preheat oven to 350 degrees F (175 degrees C). Spray a baking mold with cooking spray.
Heat oil in a saucepan over medium heat. Add onion; cook until stir until lightly browned, 6 to 8 minutes. Refrigerate until cooled completely, about 35 minutes.
Place leeks in a microwave-safe bowl and microwave until wilted, about 3 minutes.
Combine salmon, 1 cup ketchup, potato, eggs, bread crumbs, salt, pepper, and sugar in a bowl. Mix thoroughly and place in the prepared mold. Coat meatloaf with the remaining ketchup. Scatter bell pepper on top.
Bake in the preheated oven until top is browned and an instant-read thermometer inserted into the center reads at least 145 degrees F (63 degrees C), 35 to 40 minutes.