Mix the chimichurri marinade ingredients (parsley, garlic, red wine vinegar, olive oil, lemon juice, salt, and red pepper flakes) together in a small mixing bowl. Whisk until thoroughly combined.
Transfer the chicken thighs to a large mixing bowl. Pour ½ cup of chimichurri marinade over the chicken thighs and mix until the thighs are fully coated in marinade. Let the chicken thighs marinate for at least 30 minutes, or up to 2 hours. Reserve the remaining marinade.
While the chicken thighs are marinating, chop the baby potatoes into equally sized halves and quarters.
Transfer the potatoes to a large mixing bowl, drizzle with olive oil, and sprinkle with kosher salt, to taste. You only need a small amount of olive oil, just enough to give the potatoes a light coating.
Preheat your oven to 375°F. Spread the potatoes in an even layer on a large baking sheet with a rim. Nestle the chicken thighs between the potatoes.
Transfer the sheet pan to your preheated oven and bake for 35-40 minutes, or until the potatoes are tender and the chicken thighs reach an internal temperature of at least 165°F.
Optional step: During the last 5 minutes of cooking time, broil your chicken thighs and potatoes on high to give the chicken and potatoes crisp, golden brown tops.
Remove the sheet pan from the oven, drizzle the chicken thighs with the remaining chimichurri sauce, then serve and enjoy!