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 blended tomato sauce and simmer for 7-10 minutes, stirring occasionally, until sauce slightly thickens. 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 tortilla into hot tomato sauce, coating both sides thoroughly. Place on plate, fill with cheese, and fold in half or roll up. Repeat process with remaining tortillas.
05 - Arrange 3 entomatadas on each serving plate. Drizzle with additional tomato sauce, sprinkle with extra cheese, diced onion, and fresh cilantro. Top with dollop of sour cream or crema.