Preheat oven to 500ºF. Heat oil in a large deep skillet over medium heat. Add garlic and red pepper flakes and cook for 1 minute. Add spinach, tossing and cooking until it wilts, 4 to 5 minutes. Season with a pinch of pepper.
Combine ricotta, mozzarella, oregano, and pepper in a bowl. Spread over focaccia to the edges, patting into an even layer. Top with spinach mixture, spreading it around as evenly as possible. Scatter prosciutto over the top.
Bake directly on an oven rack with no baking sheet until cheese softens and crust browns and crisps, 10 to 12 minutes. Cut each pizza into six wedges.
Tip: Use mature spinach leaves here instead of baby spinach leaves, which don’t stand up to heat very well.