Cover a large griddle with aluminum foil and preheat to medium-high.

Cook tomatillos, serrano peppers, and garlic on the hot griddle, turning occasionally, until toasted and blackened — about 5 minutes for garlic, 10 minutes for peppers, and 15 minutes for tomatillos. Remove 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, in hot oil until warmed through, 3 to 5 seconds per side. Drain on a paper towel-lined plate.

Place toasted tomatillos, serrano peppers, and garlic in a blender. 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 cook at a simmer until slightly thickened, about 10 minutes.

Soak tortillas in 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 a generous amount of sauce over rolled tortillas. Top with lettuce, cilantro, crema, and cotija cheese. Pour remaining sauce on top or serve on the side.