On a cutting board, slice the chicken thighs into roughly 1-inch chunks; transfer to a large mixing bowl. Season with 2 teaspoons salt, 1 teaspoon black pepper, chili flakes and 2 of the chopped garlic cloves; toss until well combined.
Place the butter in a large skillet over medium heat. Once melted, stir in the remaining garlic and cook until it starts to sizzle, about 1 minute, then add the orzo. Cook, stirring the orzo in the garlic butter until very lightly toasted, about 3 minutes. Pour in the wine and cook, stirring, until it’s nearly absorbed. Pour in 2 cups stock, bring to a simmer, reduce heat to low, cover and cook until the orzo is al dente, about 10 minutes. Taste the orzo and season with additional salt, if needed. If almost all the liquid has been absorbed, add the remaining ½ cup stock.
Raise the heat to medium-low; stir the chicken and olives into the orzo, making sure all of the ingredients are evenly distributed. Cover and cook until the chicken is cooked through (an instant-read thermometer stuck into a piece should read 165ºF), about 5 minutes.
Stir in the spinach, lemon and tarragon. When the spinach has wilted, turn off the heat.
Top with the feta, a generous drizzle of olive oil and more black pepper, if desired, and serve hot.