In a large Dutch oven, heat butter and olive oil over medium heat. Add sliced onion and cook until translucent, about 2-3 minutes. Add garlic and cook for 1 minute, until fragrant. Add cubed beef and brown for about 3 minutes. Season meat and onion mixture with ½ teaspoon salt and ½ teaspoon pepper.
Add potatoes, carrots, and celery to the Dutch oven. Season with 2 tablespoons parsley and remaining salt and pepper. Pour in 1½ cups beef stock and cover tightly with a lid (or foil).
Bake for 1½ - 2 hours in the oven, until both vegetables and meat are very tender. When done cooking, mix the last ½ cup of beef stock with 2 tablespoons cornstarch in a small bowl to form a slurry. Remove pan from oven and stir in the slurry to thicken the stew.
Divide into bowls and garnish with remaining parsley. Serve with gluten-free French bread or gluten-free Irish soda bread.
Notes
Note: This stew is even better after it has been sitting for a bit and the flavors have had a chance to meld, so feel free to make this in advance and just reheat when ready to eat.