Trim the chicken and slice it in half lengthwise, then heat 1.5 Tbsp of olive oil in a 12″ cast iron skillet over medium-high heat for one minute.
Season the chicken with chili powder, oregano, cumin, garlic powder, and seasoned salt. Cook the seasoned chicken over medium-high heat for about five minutes a side, or until cooked through. Remove the cooked chicken to a separate cutting board.
While the chicken is browning, slice your mushrooms.
After removing the chicken from the pan, reduce heat to medium and add 1/2 Tbsp olive oil. Add the sliced mushrooms to that same skillet and cook for four minutes or until mushrooms have browned and softened, stirring occasionally and scraping up browned bits from the bottom once mushrooms start to release their liquid.
While the mushrooms are sauteing, make your avocado spread by mashing the avocado and stirring in lemon juice and sea salt. Lightly toast the bread.
Assemble your sandwiches: Add avocado spread to four of the bread slices, then top that with the cooked chicken, mushrooms, and the top slice of bread.
Try to end up with four roughly equal chicken pieces when slicing your chicken lengthwise. If not, slice it further to help even things out.
Chicken & Mushroom Sandwiches with Avocado Spread from Mashup Mom: