Add lime juice, 1 tablespoon olive oil, chopped cilantro, cumin, garlic, salt, and chicken breasts to a large zip bag. Let chicken marinate for 15-20 minutes. Meanwhile, make the salsa.
Make the Margarita salsa:
In a medium bowl, combine the cucumber, tomatoes, shallot, jalapeño (if using), and cilantro. Squeeze the lime juice over the top, season with salt, and toss to combine. Refrigerate until ready to serve.
Cook the chicken:
In a large skillet over medium-high heat, heat up the remaining olive oil.
Place the chicken breasts on a skillet and cook for 5-7 minutes on each side until the chicken is cooked through at an internal temperature of 165F. Discard chicken marinade. Remove the chicken onto a plate and let it rest for 5 minutes.
Serve:
Serve the seared chicken breasts topped with spoonfuls of the Margarita salsa.