Combine flour, milk, eggs, butter, and salt in a blender. Blend on high for 1 minute.
For the filling
Season chicken thighs generously with salt. Heat 1 tablespoon olive oil in a large pan over medium heat. Add chicken and cook until it starts to brown, turning occasionally, until it's cooked through, about 10 minutes. Transfer chicken to a cutting board and let cool slightly. Once cool, shred with forks.
Add remaining 1 tablespoon olive oil to pan. Turn heat to medium high and add red onion. Cook until it softens, about 5 minutes, then stir in garlic and asparagus, and a pinch of salt. Cook for an additional 5 minutes.
To assemble the crepes, divide filling evenly among crepes. Fold edges over the center. Place folded crepes back into pan to allow cheese to melt for about 1 minute. Garnish with scallions and serve immediately.