Preheat Blackstone Griddle to medium heat. Drizzle vegetable oil then add the chopped onion and jalapeno. Cook for 10-15 minutes until tender and caramelized.
Add the cooked onion and jalapenos to a medium mixing bowl. Pour in the tomatillos, lime juice, and salt. Blend with an immersion blender until smooth. Add the avocado and cilantro and blend again. Refrigerate until ready to serve
For the Sopes
In a medium mixing bowl, combine the Maseca, water, and salt. Knead by hand until completely smooth and incorporated. Divide the mix into 12 equal portions then roll into balls. Cover with a damp towel until ready to use.
Line a tortilla press with plastic wrap then spray with non-stick cooking spray. Press the balls of masa until they’re about 1/8” thick. Set the tortilla onto the griddle. Cook for 1 minute and 15 seconds per side. Remove from the griddle and cover with a thin towel and pinch the corners to form a barrier. Repeat with the 12 balls.
For the Chicken and Serving
Preheat Blackstone griddle to medium-high heat. Drizzle vegetable oil then add the diced chicken thighs. Season with fajita seasoning and cook for 6-8 minutes until completely cooked through.
Toast the sopes that were made earlier for 1 minute per side. Top each sope with chicken, salsa, queso fresco, and the remaining chopped cilantro. Serve immediately.