In a large pot, cook onions in olive oil over medium heat with oregano, bay leaf, garlic and salt, stirring constantly, until soft and translucent, about 15 minutes. Add tomato paste and cook 5 minutes more, then add canned tomatoes and stir constantly until sauce begins to boil. Continue cooking for 1 hour, stirring every 5 minutes so sauce does not burn. Season with salt to taste.