In a bowl, combine the chopped onion, soy sauce, brown sugar, finely chopped garlic, oil, lemon juice, tomato paste (or ketchup), salt, and pepper. If using ginger root, peel and finely chop it and add to the mixture. This is your marinade.
Place the pork cubes in a container and pour the marinade over them. Stir to coat all pieces. Cover and refrigerate for at least 30 minutes, or preferably overnight.
Preheat the oven to 180-200°C (356-392°F).
Lightly oil a baking sheet or line it with non-stick foil. Spread the drained mung bean sprouts evenly across the sheet.
Arrange the marinated pork and all its marinade over the sprouts.
Bake on the middle rack with top and bottom heat for 1 to 1.5 hours, stirring occasionally, until the pork reaches 63°C (145°F) internally.