Place the tomatoes, onion, serrano, garlic, and 2 cups water in a saucepan. Bring to a simmer, cover, and cook for 8–10 minutes, until the tomatoes soften. Turn off the heat and let the vegetables cool slightly.
Lift the vegetables into a blender and add ½ cup of their cooking liquid, the cumin, ½ teaspoon oregano, and chicken bouillon. Blend until smooth, venting the lid according to your blender instructions when blending warm ingredients. Add more reserved liquid a little at a time only if needed; the sauce should coat a tortilla.
Pour the sauce into a wide skillet and simmer gently for about 5 minutes, stirring occasionally. Taste before adding salt and pepper because the bouillon is already salty. Keep the sauce warm over low heat.
Meanwhile, heat the oil in a separate small skillet over medium heat. Briefly fry each tortilla for 10–15 seconds per side, just until flexible rather than crisp. Lift with tongs, let excess oil drain, and keep the tortillas warm. Add a little more oil if the pan becomes dry.
Dip each warm tortilla into the sauce, place it on a plate, and add about 2 tablespoons crumbled queso fresco. Fold in half. Divide the 12 entomatadas among 4 plates, spoon over remaining sauce, and add any of the optional toppings. Serve immediately while the tortillas are soft.