In a saucepan over low to medium heat, add spinach, along with a strong pinch of salt, cover, and cook until it's warmed through and released any liquid (~ 3 - 5 minutes.) Remove from the heat, drain, and set aside.
In a 10" cast-iron skillet over medium-high heat, warm the olive oil and garlic cloves, stirring until the cloves have turned a deep golden color (~ 5 minutes). Discard the garlic.
Add the spinach and a pinch of salt, and cook, stirring, so the spinach picks up all the garlicky oil, about 1 minute. Transfer the spinach to a plate.
Wipe out the pan and preheat the broiler.
In a medium bowl, whisk the eggs and milk.
Add half of the mozzarella, half of the Parmesan, a few generous pinches of salt, and two or three grinds of black pepper. Whisk until thoroughly incorporated.
Add the spinach to the cheesy egg mix. Whisk again to combine
Using the same pan over medium heat, melt the clarified butter until it starts to foam but doesn’t start to brown. Briefly re-whisk the egg mixture and pour it into the pan. Give the batter a few stirs with a rubber spatula to make sure the spinach is evenly distributed
Decrease the heat to low and let the frittata cook, shaking the pan occasionally until only the surface remains runny (~8-10 minutes). The edges should be completely set, and the interior should be thickened and custardy.
Dot the top of the frittata with the ricotta and sprinkle it with the remaining mozzarella and Parmesan.
Transfer the pan to the oven and broil for about 2 minutes, until the top sets and the cheese melts and forms a golden brown crown.
Transfer the frittata to a wire rack and let cool until warm or at room temperature. Slice and serve