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 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 as needed.
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 thoroughly. Place on a plate, add cheese filling, and fold in half or roll up. Repeat 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 a dollop of sour cream or crema.