Parboil the cubed potato in boiling water for 5 minutes until slightly tender. Drain and set aside.
Heat oil in a large skillet or wok over medium-high heat. Add massaman curry paste and red curry paste, frying for 1-2 minutes until fragrant and sizzling.
Add sliced onion, minced garlic, and minced ginger to the pan. Cook for 2-3 minutes until softened and aromatic.
Add shaved beef in batches, stirring frequently to brown lightly on all sides, about 2-3 minutes total.
Pour in coconut milk, beef broth, fish sauce, soy sauce, and brown sugar. Add Thai chile, chili flakes, cinnamon stick, and bay leaf if using. Stir to combine.
Add the parboiled potatoes, sliced carrots, and roasted peanuts to the curry. Bring to a gentle simmer.
Reduce heat to medium-low and simmer for 12-15 minutes, stirring occasionally, until potatoes are tender and sauce has thickened.
Taste and adjust seasoning with more fish sauce for saltiness, sugar for balance, or chili oil for extra heat.
Remove cinnamon stick and bay leaf. Squeeze lime juice over the curry and garnish with fresh cilantro and extra chiles or chili oil if desired. Serve immediately over jasmine rice.
Notes
Storage tip: Keeps in refrigerator for up to 4 days and flavors improve over time. Reheat gently with a splash of broth or coconut milk. Can be frozen for up to 3 months. Adjust heat level by reducing or omitting the Thai chile and chili flakes for a milder version.