No need to take out the blender or food processor, easiest way to make peach puree is to take a fine cheese grater and grate the peach meat. (Not the skin.)
Add peach puree into a small mixing bowl.
Add soy sauce, brown sugar, pressed garlic, and fresh grated ginger. Whisk together and set aside.
Chicken:
Preheat the oven to 375.
Preheat cast iron skillet (or another oven-safe cooking pan) over medium-high heat.
Slice onions and a peach thinly. Set aside.
Add a little oil to the cooking pan and place chicken thighs in it. Sear for about 4-5 minutes on each side and take the chicken out of the pan.
Add sliced onions and peaches to the pan. Cook for a few minutes until softened.
Add chicken thighs back to the pan and nestle it with peaches and onions in between.
Spread sauce all over chicken and around. Gently shake to spread sauce evenly.
Place pan into the oven and bake for 35-40 minutes, until chicken is completely cooked. (Time will depend on thickness of chicken thighs.)