Step 1: Season the cubed steak generously with salt and pepper. In a large Dutch oven, heat a tablespoon of oil over medium-high heat and sear the steak in batches until browned on all sides. Remove the steak and set aside.
Step 2: Lower the heat to medium and add the butter to the same pot. Sauté the onion, carrots, and celery for about 5-7 minutes until softened.
Step 3: Add the minced garlic and dried thyme, cooking for another minute until fragrant. Sprinkle the flour over the vegetables and stir constantly for 2 minutes to create a roux.
Step 4: Slowly pour in the beef bone broth while whisking to prevent lumps. Use a wooden spoon to scrape the browned bits (fond) off the bottom of the pot.
Step 5: Add the diced potatoes to the pot. Bring to a gentle boil, then reduce heat and simmer for 15-20 minutes, or until the potatoes are fork-tender.
Step 6: Stir in the heavy cream and return the seared steak to the pot. Simmer for an additional 5 minutes to heat the steak through and thicken the soup. Adjust seasoning with salt and pepper as needed.
Step 7: Ladle the soup into bowls and garnish with fresh chives before serving.