Add beef, mushrooms, tomatoes, tomato paste, herbs, thyme, onion powder, and remaining stock. Bring to a boil, cover, and simmer for 1 hour 45 minutes.
In the last 20 minutes, stir in beans. Mix cornstarch with water, add to casserole, and cook until thickened. Season with salt and pepper, garnish with parsley.
Notes
For a richer flavor, let the casserole rest for a few minutes before serving.