Heat 1 tablespoon of vegetable oil in a large heavy bottom pan (I like cast iron for this) over medium high heat until smoking lightly. Add half of the sirloin cubes to the pan, spreading them into an even layer. Cook for about 1-2 minutes per side, flipping when the first side has browned and removing from pan when the second has as well. Repeat with the other half of the sirloin, adding the other tablespoon of oil before cooking.
Turn heat down to medium and add 1 tablespoon of oil, onion, carrots, and celery to the pot. Add ½ teaspoon of salt the vegetables and cook, stirring occasionally, until onions start to soften ~5 minutes. Mix in garlic and cook for an additional minute.
Turn the heat back up to med-high, then pour in sherry to deglaze the pan, scraping browned bits from the bottom of the pan. Cook sherry for about 2 minutes, until it has thickened. Add broths, fish sauce, bay leaf, rosemary, and thyme to the pot and bring to a simmer. Mix the beef back into the pot. Turn pot down to low, then cover and cook for 2 ½ hours.
Stir potatoes into the stew, then recover the pot and cook for 30 minutes.
Remove top from the pot, turn heat up enough to maintain a light simmer, and cook for 15 minutes. Stir in peas and cook for an additional 5 minutes. Remove bay leaf and thyme sprigs. Season stew with ½ teaspoon each of salt and pepper, adjusting as necessary for your taste. Serve right away with some fresh parsley or freeze for later.