Season the chicken with salt and pepper and squeeze 1/2 lemon over the chicken breasts, being careful to remove seeds.
Grill the chicken until cooked all the way through, about 5 minutes on each side. If chicken breast is thick, slice in half for quicker cook times.
While the chicken is cooking, sautee the chopped onions in 1 tbsp on oil until they soften and add the diced tomatillos, pepper sauce, parsley, cilantro, 1/2 tsp salt, and half the serrano pepper and stir. Cook for 5 minutes until the tomatillos soften. Add tomatillo mixture to a blender and puree ingredients.
Add cooked chicken to a baking dish and cover completely with the tomatillo salsa. Top with cheese and remaining serrano chile slices on top and bake chicken in a 350 pre-heated oven for about 5 minutes until the cheese has melted and is bubbling. Remove from oven and top with cilantro.
While the chicken is baking, make the tortillas by adding corn flour, 1/4 tsp of salt in a bowl and slowly adding 3/4 cup of hot water. Stir together until dough forms then roll about 2-3 tbsp of dough in hand to shape like a ball. Flatten ball with the palm of your on a floured board. Lay a piece of parchment paper on top and roll until desired thickness. Makes about 6 tortillas.
Heat up each tortilla in a heavy cast iron pan with a pat of butter. Heat on each side until tortillas are a little browned on both sides.