Heat olive oil in a large Dutch oven over medium-high heat. Working in batches, sear the cubed beef until deeply browned on all sides, about 8-10 minutes total. Don't skip this step—the crust develops incredible flavor through caramelization that defines the entire casserole.
Remove beef and set aside, then add diced onion and celery to the pot. Sauté for 3-4 minutes until softened and fragrant. Stir in minced garlic and tomato paste, cooking for another minute until the paste darkens slightly and becomes aromatic.
Pour in beef broth, scraping up any browned bits from the bottom—these bits are liquid gold flavor. Return beef to the pot along with carrots, potatoes, thyme, rosemary, bay leaf, salt, and pepper. Bring to a simmer, then reduce heat to medium-low and cover partially.
Simmer this hearty beef casserole crowd favorite for 35-40 minutes until vegetables are fork-tender and beef is melt-in-your-mouth soft. Stir occasionally to ensure even cooking and prevent sticking on the bottom.
Add sliced mushrooms and cook uncovered for 5 more minutes. In a small bowl, whisk cornstarch and water until smooth, then stir into the simmering liquid. Cook for 2-3 minutes, stirring gently, until the sauce thickens beautifully and coats the back of a spoon.
Taste and adjust seasoning with additional salt and pepper if needed. Remove the bay leaf and serve hot, garnishing with fresh parsley if desired.