Heat a dry pan over medium heat and toast your rice flour for about 5 minutes, stirring constantly until it turns light golden brown. Take it off the heat and set aside.
Heat oil in a large pot over medium heat. Add your minced garlic and cook until fragrant, about 30 seconds. Add the sliced onions and cook until they become soft and clear, about 2-3 minutes.
Put the chicken pieces in the pot and cook until they're lightly browned on all sides, about 5-7 minutes. Add fish sauce and cook for another 2 minutes.
Mix the annatto powder with water in a bowl until well combined. Pour this red mixture into the pot and bring everything to a boil. Once boiling, lower the heat, cover the pot, and let it simmer until the chicken becomes tender, about 15-20 minutes.
Add the cut eggplant and string beans to the pot. Cook them until they're tender but still have a slight crunch, about 5-7 minutes.
Add the peanut butter and stir until it's completely mixed into the sauce. In a separate bowl, mix your toasted rice flour with one cup of hot broth from the pot, making sure there are no lumps. Slowly pour this mixture into the pot while stirring constantly.
Add salt and pepper to taste. Put in the pechay leaves and cook just until they wilt, about 2-3 minutes. The sauce should be thick enough to coat your spoon but still be pourable.
Let everything rest for 5 minutes before serving. Serve hot with steamed rice and shrimp paste on the side.
Remember to cut your vegetables roughly the same size so they cook evenly. If your sauce becomes too thick, you can add a little hot water. If it's too thin, let it simmer uncovered for a few extra minutes.