In a bowl, combine 500 g minced meat, salt, pepper, curry, 1 teaspoon medium-hot mustard, and 1 teaspoon sweet mustard. Mix until well combined. Refrigerate for at least 15 minutes (or up to 2 hours).
Shape the meat mixture into meatballs roughly 2 cm in diameter, roll in flour to coat lightly, and set aside.
Halve 3 large onions, slice thinly, roll in remaining flour to coat lightly, and set aside.
In a baking dish on the stove over medium-high heat, bring 500 ml water to a boil. Stir in 5 tablespoons tomato paste, 2 teaspoons medium-hot mustard, 1 teaspoon sweet mustard, salt, pepper, marjoram, curry, garlic powder, and sweet paprika powder. Simmer for 2 minutes, then add 100 ml cream and stir until combined.
In a separate large pan over medium-high heat, fry the meatballs on all sides until browned, about 10 minutes. Transfer to the sauce in the baking dish.
In the same pan, fry the onions over medium-high heat until lightly browned, about 8 minutes. Pour into the baking dish over the meatballs.
Bake at 200°C (392°F) for 20 minutes until meatballs are cooked through.