HEAT 1/3 cup corn oil in a medium saucepan over medium-low heat; CHOP 1 medium onion together with 4 cloves of garlic in a food processor; SAUTE the onion and garlic mixture in the corn oil until translucent and golden, and almost caramelized.
ADD 1/3 cup flour to the saute mixture; STIR the mixture as it cooks, until it is deep golden in color (mixture will be paste-like).
PLACE into the large bowl of a food processor enough fresh ripe Roma tomatoes to yield approximately 1 1/2 cups pureed, and one stem-removed seeded fresh jalapeno pepper; PROCESS both until pulverized.
TRANSFER the sauteed onion and garlic roux from the saucepan into the bowl of the food processor with the tomatoes and jalapeno, submerging the into the pulverized mixture first.
PROCESS until all ingredients until finely pureed.
POUR pureed mixture into the same medium saucepan.
STIR in 5 cups water, 2 teaspoons salt, 1 1/2 teaspoons crushed Mexican oregano (crush between fingers) 1/4 teaspoon finely ground black pepper, and 1 small bay leaf.
BRING mixture to boiling while whisking; REDUCE heat to medium and cook for about 45 minutes, whisking occasionally, or until desired consistency is reached; COVER slightly vented so steam can escape; KEEP sauce warm over lowest heat, whisking occasionally until served, or reheat and stir before serving.
POUR a small pool of warmed sauce onto chile relleno serving plate; PLACE chile relleno over sauce; TOP chile relleno with additional sauce; GARNISH as desired; SERVE.