For the Filling:
Sauté the onion with ¼ cup of oil over low heat. When the onion becomes soft and translucent, add the serrano chile and cook for another minute. Remove the onions and chile from heat and stir in the epazote and squash blossoms. Set aside.
Preheat a skillet over high heat until very hot. Working in batches, slowly add oil and sear the yellow squash. When the yellow squash begins to brown, about 45 seconds, flip it over and cook the other side. Roast the Poblano chile over an open flame until the skin is charred. Place the Poblano in a plastic covered bowl and allow it steam for 10 minutes. Peel and discard the skin and cut the flesh into narrow strips. Mix the onion, poblano and squash mixtures together. Season to taste.
For the Salsa de Chile Serrano:
Char all ingredients on a comal. Place all ingredients in a blender and puree until smooth. Season with salt.
For the Quesadilla:
To build the quesadillas start with open tortillas. On one half of each tortilla, add the squash-rajas mixture, then squash blossoms, Chihuahua cheese, and epazote.
Fold the tortilla over the filling. Cook the quesadilla over high heat in a nonstick pan for about 3 minutes.
Flip and cook the other side for another 3 minutes or until the cheese is melted.
Serve the warm quesadilla with the salsa serrano and sweet onions.