Add the lemon slices in a single layer to an oven-safe skillet, and top with the butter and garlic. Cook on the stove over medium-high heat until the butter is melted and the garlic is fragrant, about 2-3 minutes. Remove from heat and set aside. If you don't have an oven-safe skillet, transfer to an oven-safe baking dish.
Preheat the oven to 400 degrees F.
Add the ground chicken, egg, breadcrumbs, parmesan, onion powder, garlic powder, salt and pepper to a bowl. Mix until just combined.
Roll the chicken mixture into 2 tablespoon-size meatballs and place in the skillet on top of the lemon slices (will make about 18 meatballs). Loosely cover the meatballs with tinfoil and bake for 10. Remove the tinfoil and continue baking another 15 minutes, or until golden brown and cooked through.
Meanwhile, make the orzo. Heat a skillet or pot over medium-high heat with butter and garlic. Cook for 2-3 minutes. Pour in the broth, milk, lemon juice, salt and pepper. Bring to a boil, then reduce heat to medium-low and mix in the orzo. Cover and simmer for about 10 minutes, or until the orzo is al dente, stirring frequently so the orzo doesn't stick. Add more broth or milk, if needed.
Mix the parmesan and baby spinach into the orzo until the cheese is melted and the spinach is wilted.
Serve the meatballs over the creamy spinach orzo and drizzle with the extra lemon butter.