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