Trim chicken and season on both sides with salt and pepper, then place in small nonreactive baking dish.
Combine vinaigrette, lemon juice, garlic, and oregano.
Pour over chicken, then marinate in the refrigerator for at least half an hour.
Using enough olive oil to cover the bottom of a large pan, heat 2-3 minutes over medium heat before adding chicken.
Cook chicken over medium heat 8-12 minutes a side, depending on thickness, until it reaches an internal temperature of 165 degrees.
Let cool, then slice into strips.
Arrange 4 oz spinach on a platter, then decorate your leafy "tree" with chicken strips, feta crumbles, dried cranberries, olives, and onion.
Garnish with oregano, and serve with additional dressing to taste.
The ingredients listed make enough for two festive "trees" as shown here, or just create one finished salad for inspiration and lay out the rest of the ingredients so that everyone can assemble their own to taste. I also added a little "star" to the top of mine that I cut out of Monterey Jack cheese (optional).