Heat oil in saucepan over medium-low heat. Sauté shallot in oil 2 minutes, or until beginning to soften. Add garlic and red pepper flakes, and sauté 1 minute more, or until just fragrant. Whisk in peanut butter, coconut milk, soy sauce, brown sugar, and 1/2 cup water until smooth, and bring to a boil. Reduce heat to medium-low, and simmer 5 minutes. Stir in lime juice. Serve hot, or at room temperature.