Marinate the chicken: In a small bowl, whisk the olive oil, vinegar, mustard, oregano, thyme, and rosemary. Pour the mixture into a large resealable plastic bag and add the chicken. Squeeze as much air from the bag as you can, seal the bag, and gently toss to coat the chicken in the marinade. Refrigerate for at least 30 minutes and up to 24 hours.

Make the cucumber mint salsa: In a small bowl, combine the cucumber, tomatoes, and mint. Add the lemon juice and olive oil, stir, and set aside until you are ready to serve. The salsa can be made 1 day ahead of time and stored, covered, in the refrigerator.

Blanch the broccoli rabe: Bring a large pot of water to a boil. Add the broccoli rabe and cook until bright green and just tender, 2 minutes. Use a slotted spoon to remove the broccoli rabe and transfer to a paper towel-lined plate.

Make the fajitas: Set a large nonstick skillet over medium-high heat. Remove the chicken from the marinade, discarding the remaining marinade, and add the slices in a single layer. Cook, stirring occasionally, until browned and cooked through, 6 to 8 minutes. Transfer the chicken to a plate. Add the olive oil, red onion, and broccoli rabe and cook, stirring frequently, until tender, 4 to 6 minutes. Return the chicken to the pan and stir in the parsley.

Warm the tortillas: In a separate large, dry skillet over medium-high heat, warm the tortillas (2 or 3 at a time) for 2 minutes, using tongs to flip them halfway.

To serve: Place 2 tortillas on each of 4 plates and divide the chicken and vegetable mixture among them. Top each taco with crumbled feta and cucumber mint salsa. Serve immediately.