01 - Boil tomatoes in a pot of water for 5-6 minutes until skins split. Drain and transfer to a blender with garlic, onion, serrano chilies if using, cumin, salt, and pepper. Blend until completely smooth.
02 - Heat 2 tablespoons vegetable oil in a skillet over medium heat. Pour in the blended tomato sauce and simmer for 7-10 minutes, stirring occasionally, until slightly thickened. Adjust seasoning to taste.
03 - Heat a small amount of oil in a separate skillet over medium heat. Lightly fry each tortilla for about 10 seconds per side just until soft and pliable. Drain briefly on paper towels.
04 - Dip each fried tortilla into the hot tomato sauce, coating both sides completely. Place on a plate, fill with cheese, and fold in half. Repeat with all tortillas, arranging 3 per serving plate.
05 - Drizzle additional tomato sauce over the arranged entomatadas. Sprinkle with extra cheese, diced onion, and fresh cilantro. Top with a dollop of sour cream or crema.