Marinate beef with salt, black pepper, white pepper, soy sauce, and flaxseed oil for 30 minutes.
Toast bread until golden brown.
Sauté garlic and onion in butter for 5 minutes over low heat.
Add marinated beef and brown sugar. Mix well to coat the meat.
Melt cheese slices on top of the cooked beef mixture.
Place melted cheese slices on the toasted bread and cover with another slice of toast.
Serve immediately.