PREPARING THE CHICKEN
In a small bowl, mix the oregano, salt, and pepper. Season the chicken thighs and rub them with the olive oil. Let sit at room temperature for 20 minutes.
Heat a grill pan or skillet over high heat. Sear and cook the chicken thighs until done. Let cool, then slice them.

PREPARING THE LEMON VINAIGRETTE
In a small bowl, whisk together all the vinaigrette ingredients.

PREPARING THE SALAD
In a large salad bowl, combine the lettuce, spinach, and mushrooms. Toss with the lemon vinaigrette.

Serve immediately.