Line 2 baking sheets with parchment paper and preheat the oven to 425℉.
In a medium saute pan, cook the bacon over medium-low heat until it begins to crisp about 3-5 minutes. Add the diced onions and cook until they soften about 3 minutes. Season with salt, pepper, and thyme. Set aside to let cool.
In a medium-sized mixing bowl, combine the ricotta cheese, sour cream, and egg yolk. Stir in more salt and pepper to taste and set aside.
Lightly flour your counter. Lay one puff pastry sheet flat and roll it into a thin 9 x 13-inch rectangle. Gently place on a prepared baking sheet and repeat the steps with the second puff pastry sheet.
Use a fork to pierce (or dock) the pastry sheets slightly, leaving a 1-inch border near the edge.
Divide the cheese sauce between both tarts and spread evenly. Then top each tart with half of the bacon and onion mixture.
Place the tarts in the oven and bake, rotating halfway through until the crusts are golden brown, about 25 minutes.
Garnish with fresh grated Parmesan cheese and chives.
Notes
*Note: The original recipe called for Farmer's cheese and creme fraiche, but I swapped both for more easily accessible ingredients like ricotta and sour cream.