Preheat oven to 450°F. Also preheat a large non-stick pan over medium-high heat.
To the pan add the olive oil, onions, thyme, salt, and pepper. Cook eight minutes until soft and caramelized, stirring often.
While the onions cook, trim root ends of the brussels sprouts and pick leaves to separate them. Keep trimming the ends as needed to release more leaves until you reach the core of each. Set aside.
Add brussels sprout leaves and cook another two minutes. Remove from heat.
In a small bowl combine the ricotta, Parmesan and salt. Spread generously over each flatbread, leaving a two centimeter border around the perimeter for easy eating. Scatter the caramelized onion mixture over the top and transfer to a parchment-lined baking sheet. Bake five to seven minutes until hot and crispy around the edges. Slice as desired, transfer to a serving platter, and enjoy.