Ricotta Stuffed Squash Blossoms

In a large bowl of cold water, carefully rinse blossoms and remove all stem ends, pistils or stamens, then gently shake dry. In a medium bowl, stir together ricotta, egg yolk, salt, pepper, garlic, herbs, red pepper flakes, and lemon zest until smooth. Using an offset spatula or small spoon, stuff each blossom with about 2 teaspoons ricotta mixture then press the petals shut and twist slightly to seal in the filling. In a large bowl, whisk together cornstarch and baking powder. Place egg white in a small bowl and whisk until slightly foamy. Brush each blossom with a thin layer of beaten egg white, then roll in cornstarch and shake off any excess starch.  In a large skillet over medium heat, heat oil until shimmering. Add battered blossoms in a single layer, then fry, turning occasionally, until golden on all sides, 4 minutes. Transfer to a plate. To make quesadillas: Return skillet to medium-low heat, then add a tortilla and top evenly with ¼ cup mozzarella and 3 fried squash blossoms. Once cheese begins to melt, fold quesadilla in half and continue cooking until both sides are golden. Repeat with remaining tortillas, mozzarella, and squash blossoms.