1. Heat 2/3 cup olive oil in a large pot. Add the chopped onion and cook until soft, 6 to 8 minutes. Add the yellow split peas and toss to coat with the olive oil. Stir for 1 to 2 minutes. Add enough water to cover split peas by 5 cm (2 inches). Cover and bring to a boil over medium heat. Reduce heat, uncover, and cook over very low heat, stirring occasionally to keep from sticking, for 1 ½ to 2 hours, until the split peas are completely disintegrated. During the course of cooking, add water.

2. When the split peas have reached the consistency of loose mashed potatoes, remove from heat and add salt, pepper, oregano and vinegar to taste. Cover with a cloth and let sit for 1 to 2 hours to cool and solidify. Spread on a plate or platter, sprinkle with raw onions and drizzle generously with remaining olive oil.