Preheat the oven to 350 degrees F (175 degrees C).

Place butter into a 9x13-inch baking dish and melt in the oven while it preheats, about 5 minutes.

Stir onion, basil, red pepper flakes, salt, and black pepper into melted butter. Add chicken thighs, turning to coat with onion mixture and ending skin-side up.

Bake in the preheated oven until no longer pink at the bone and the juices run clear, about 1 hour. An instant-read thermometer inserted near the bone should read 165 degrees F (74 degrees C). Remove chicken from the dish.

When the chicken is nearly finished cooking, bring a large pot of lightly salted water to a boil. Cook pasta in boiling water until tender yet firm to the bite, about 11 minutes; drain.