In a mixing bowl, combine the ground beef, minced onion (if using), fish sauce, and black pepper. Mix until just combined and form into 4 patties. Cover with plastic wrap and refrigerate for about 30 minutes.

In a skillet over medium heat, add a drizzle of oil. Add the julienned onions, sprinkle with a pinch of salt, and sauté until soft and caramelized—about 8-10 minutes. Remove from skillet and set aside.

Return the same skillet to heat and add more oil. Lay potato slices in a single layer, cover, and cook for 2 minutes. Flip and continue to cook until golden brown and tender—about 4-5 minutes. Remove and set aside.

In a pot, combine 1/3 cup of coconut milk with Massaman curry paste. Cook over medium heat until thick and fragrant. Stir in remaining coconut milk, brown sugar, tamarind, and simmer for 2 minutes. Toss in peanuts, taste, and adjust with fish sauce if needed. Keep warm.

Heat a grill or pan over medium-high heat. Add the burger patties and cook for 3-4 minutes on each side for medium doneness.

Toast the buns on the grill for a minute during the last few minutes of cooking, if desired.

Serve by dipping patties into warm curry sauce, placing them onto lettuce-lined buns. Top with crunchy potato slices, sautéed onions, and pickles if desired.