Line a large griddle with aluminum foil and preheat to medium-high.
Toast tomatillos, serrano peppers, and garlic on the hot griddle, turning occasionally until blackened — about 5 minutes for garlic, 10 minutes for peppers, and 15 minutes for tomatillos. Transfer to a bowl and allow to cool.
Heat oil in a small, deep skillet to 350 degrees F (175 degrees C).
Lightly fry tortillas one at a time until warmed through, about 3 to 5 seconds per side. Drain on a paper towel-lined plate.
In a blender, combine toasted tomatillos, serrano peppers, and garlic. Add water and blend until smooth. Pour into a saucepan over medium heat and bring to a boil. Dissolve chicken bouillon in the mixture, reduce heat, and simmer until slightly thickened, approximately 10 minutes.
Dip tortillas in the sauce one at a time for a few seconds. Fill each tortilla with shredded chicken and sprinkle with sauce. Roll up tortillas and place seam-side down in a serving dish.
Spoon generous amounts of sauce over rolled tortillas. Top with lettuce, cilantro, Mexican crema, and cotija cheese. Optionally, pour remaining sauce over the enchiladas or serve it on the side.