Heat oil:
Warm 1 tablespoon olive oil in a large sauté pan or 5-quart braising pan over medium-high heat.Coat beef in flour:
In a large shallow bowl, whisk together flour, garlic powder, celery salt, and several generous pinches of salt and pepper. Coat beef cubes in the flour mixture.Brown beef batches:
Place half the beef into the pan and brown on all sides. Add the remaining 1 tablespoon olive oil to the pan and sear the second batch of beef. Move all beef to the slow cooker insert.Sauté onion and celery:
Without cleaning the pan, add butter and let it melt over medium heat. Toss in onion and celery with a couple pinches of salt and pepper. Sauté, stirring now and then, for about 4 minutes.Cook garlic and tomato paste:
Stir in garlic, tomato paste, and dried thyme. Cook, stirring often, for 1 minute.Deglaze with Worcestershire and vinegar:
Pour in Worcestershire sauce and balsamic vinegar to deglaze the pan, scraping up the browned bits from the bottom.Transfer to slow cooker with vegetables:
Transfer the onion mixture to the slow cooker. Add carrots, potatoes, beef stock, bay leaves, and a few generous pinches of salt and pepper. Gently mix everything together.Cook on high or low:
Cook on high for 4-5 hours or on low for 8-10 hours.Thicken with cornstarch slurry:
Mix 3 tablespoons cornstarch with 3 tablespoons cold water. Stir the slurry and frozen peas into the stew and cook on high for 1 hour.Finish and garnish:
The stew is ready when potatoes are fork-tender and meat falls apart easily. Adjust seasoning with salt and pepper. Sprinkle fresh parsley on top before serving.