In a food processor, combine red onion, mild red chillies, ground coriander, ground cumin, ground cinnamon, white pepper, garlic cloves, lemongrass, minced ginger, and shrimp paste if using. Blend until smooth, ensuring a fragrant paste forms, about 2 minutes. Once blended, set aside in a bowl.

Take beef chuck and place it in a large mixing bowl. Sprinkle cornflour, salt, and freshly ground black pepper over the meat. With clean hands, toss the beef until evenly coated.

Heat a large, heavy-bottomed skillet or Dutch oven over medium-high heat and add vegetable oil. Once the oil shimmers, carefully add the coated beef in batches, ensuring pieces are spaced out. Sear for about 5 minutes per side, or until a rich, golden crust forms.

Reduce the heat to medium and stir in your prepared Massaman paste into the pan with the seared beef. Cook for 2-3 minutes, allowing the paste to bloom and release its fragrant oils.

Pour in the beef stock and canned coconut milk, stirring thoroughly to combine all ingredients. Bring to a gentle simmer, then cover and let it cook on low heat for 1 hour and 45 minutes.

Stir in the halved baby new potatoes. Mix them gently into the curry, ensuring they are submerged in the sauce. Cover the pot again and continue to cook for an additional 25-30 minutes.

Once everything is tender, remove the pot from heat and squeeze in fresh lime juice to brighten the flavors. Serve hot, garnished with freshly chopped coriander leaves, slices of red chillies, and lime wedges.