Combine the parsley, cilantro, chives, garlic, red pepper flakes, salt, and pepper in a food processor. Pulse 8-10 times until finely chopped but not pureed; you want texture, not a paste. No food processor? No problem; finely chop everything by hand with a sharp knife and stir together in a bowl. Transfer to a bowl, stir in the red wine vinegar, then stream in the olive oil and stir to combine. Taste and adjust salt.

Spoon half the chimichurri into a separate bowl and cover tightly; this is your serving sauce and it stays pristine. Season the chicken thighs generously on both sides with salt and pepper, add them to the remaining chimichurri, and toss to coat. To marinate, you have two options: at room temperature for 30 to 60 minutes, or covered in the refrigerator for up to 8 hours. The room temperature method is great if you're grilling right away. If you're prepping ahead, the fridge is your friend. Either way, pull the chicken out 30 minutes before grilling so it is not cold when it hits the grates.

Heat a gas or charcoal grill to medium-high. Once hot, brush the grates with a folded paper towel dipped in neutral oil for a clean, well-oiled surface.

Remove the chicken from the marinade and shake off any excess; large herb pieces on the surface will burn rather than char. Lay the thighs flat on the grill and cook undisturbed for 5-6 minutes until they release easily with good char marks. Flip and cook another 4-5 minutes until cooked through and the internal temperature reads 165°F (74°C).

Transfer to a cutting board and rest for 5 minutes. Slice against the grain or serve whole, arrange on a platter, and spoon the reserved chimichurri generously over the top. Put the rest of the chimichurri on the table.