Make peanut sauce. In a medium bowl, whisk together coconut milk, peanut butter, brown sugar, soy sauce, fish sauce, and juice of ½ lime until smooth. Set aside.
Slice beef into ¼-inch strips. Toss with water, cornstarch, and oil until well combined. Let sit for 10 minutes.
Meanwhile, make stir fry sauce. Combine water, soy sauce, ketchup, oyster sauce, Thai sweet chili sauce, and sesame oil until well combined.
Heat 1 tablespoon oil in a large saute pan over medium-high heat. Add beef and arrange in an even layer. Cook until the beef is about 80% cooked through. Remove and set aside.
In the same pan, add the remaining tablespoon of oil. Add red bell pepper and onion, and saute until vegetables are softened, about 3-4 minutes. Add garlic and ginger and saute for 30 more seconds. Add prepared sauce and bring to a simmer. Continue to cook for 1 minute.
Add beef to the pan along with any juices, tossing to combine. Simmer for another minute, and or until beef is hot. Remove from heat, drizzle peanut sauce on top, and garnish with sesame seeds. Serve with rice and pickled cucumbers.