Place beef in a large bowl. Pour lemon juice, tamari, garlic and season with sea salt and black pepper. Mix. Add the sliced onions. Mix and let it sit for 20 minutes.
Heat a pan over medium high heat. Add the oil and cook the marinated meat in batches over medium heat. Cook for 1 - 2 minutes on each side. Transfer to a bowl to cook the next batch.
Using the same pan, Add the onions and cook until softened and browned. About 5-7 minutes. Remove from heat and set aside.
Place beef back in the pot and add half of the onions. Add water. Bring to a boil and reduce heat to low.
Cover and simmer for 35-45 minutes until beef is fork tender.
Add the remaining onions set aside earlier and mix well before serving.
If you can find fresh calamansi from an Asian or Filipino store, substitute lemon juice with 3/4 cup calamansi juice.