Preheat oven to 375.
In a mixing bowl, combine ricotta, chopped garlic, eggs, cheese, zest, lemon juice, salt and pepper; with a mixer on low, mix until combined and set aside.
Heat olive oil in a frying pan and add cubed chicken pieces; cook for 4 minutes or until heated through.
Add spinach leaves and continue to cook until spinach is wilted; about 2 minutes.
Remove from heat and transfer 1/2-cup of chicken mixture to ricotta mixture; stir to combine.
Add the remaining chicken mixture to the ricotta mixture and mix until thoroughly incorporated. Set aside.
Brush a cast iron skillet with a little bit of the melted butter.
Unfold phyllo sheets.
Place a phyllo sheet on the bottom of the cast iron skillet.
Brush with melted butter.
Top with seven more sheets, brushing each sheet with melted butter before adding the next.
As you work, keep the unused stack of phyllo sheets covered so they don't dry up.
Spread the chicken-ricotta mixture over the phyllo sheets.
Top with remaining 8 phyllo sheets, brushing each sheet with melted butter, including the top sheet.
Bake for 30 to 35 minutes, or until top is browned and flaky.
Let stand 10 minutes.
Cut and serve.