Make the honey mustard sauce. Preheat oven to 400°F (205°C). Combine Dijon, honey, and mayonnaise in a small bowl, then whisk to combine. Set aside.
Dredge the chicken in flour. Combine flour, onion salt, and black pepper in a large Ziplock bag. Add chicken cutlets, seal, then shake to thoroughly coat.
Brush with honey mustard. Brush the honey mustard on one side of each chicken cutlet. Combine finely ground pecans and panko breadcrumbs in a large bowl, then stir to mix well. Pour half of the breadcrumb coating on top of chicken, pressing down gently to ensure it sticks. Turn chicken over and repeat until all cutlets are coated on both sides.
Heat the skillet. Heat a large skillet over medium heat. Add butter and oil. When hot, add 2 chicken cutlets and cook for 3-4 minutes, or until golden brown. Carefully flip, then cook chicken an additional 3 minutes. Transfer seared chicken to a rimmed baking sheet, wipe out the pan (if needed, and possibly add a bit more oil), then cook remaining chicken cutlets.
Finish cooking in preheated oven. Place baking sheet with chicken in a warm oven and cook another 5-10 minutes. Chicken is fully cooked when it reaches an internal temperature of 165°F (74°C). Serve with lemon wedges, microgreens (optional, but recommended!), and a sprinkle of flaky sea salt.