Prepare Chimichurri Sauce:

Combine all ingredients for the chimichurri sauce in a bowl in the order listed.
Stir together, cover, and refrigerate for 2 hours or overnight to let flavors meld.
Marinate the Chicken:

In a large mixing bowl, combine chicken thighs, 1/2 tsp salt, cubed tomato, finely chopped red onion, and half of the prepared chimichurri sauce.
Toss until chicken is evenly coated. Cover and marinate for 15 minutes at room temperature or refrigerate overnight if preparing ahead.
Grilling the Chicken:

Preheat the grill over medium/high heat.
Place chicken smooth-side-down on the hot grates, cover, and grill for 5-7 minutes per side.
Ensure the internal temperature reaches 165˚F at the thickest point with an instant-read thermometer.
Sautéing the Chicken:

Preheat a heavy skillet over medium heat and add 1 Tbsp olive oil.
Add chicken smooth-side-down and sauté for 5-7 minutes per side or until cooked through.
Ensure the internal temperature reaches 165˚F at the thickest point.
Serving:

Arrange the cooked chicken on a platter.
Spoon the remaining chimichurri sauce over the chicken.
Serve immediately and enjoy!
Enhance your Chimichurri Chicken with the vibrant flavors of tomato and red onion!