Heat 1 tablespoon of olive oil in a large skillet over high heat. Add the ground beef, season with 1/2 teaspoon each of salt and pepper, and cook, breaking it apart, until fully browned. Remove from heat and set aside.
Meanwhile, in a large soup pot, add 1 tablespoon of olive oil and 1 tablespoon of butter to melt over medium-high heat. Add diced onion, season with 1/2 teaspoon each salt and pepper, and sauté for 5-7 minutes until tender. Add garlic, parsley, basil, mustard powder, and potatoes; sauté for 1 minute.
Add the beef stock or broth and bring it to a boil. Then, reduce the heat to medium-low, cover, and let it simmer for 15-20 minutes, or until the potatoes are fork tender.
In a separate pot, melt the remaining 4 tablespoons of butter over medium heat. Add the flour and whisk continuously for 1 minute. Gradually whisk in the milk, stirring constantly, until the mixture thickens. (It should be thick enough to coat a wooden spoon.) Lower the heat, then add the heavy cream, sour cream and Worcestershire sauce, whisking until smooth and well combined.
Pour the cream mixture and cooked beef into the soup, stirring to combine. Gradually add the shredded cheese, one handful at a time, stirring until fully melted. Taste and adjust seasoning with additional salt and pepper, if needed.
For an optional finishing touch, garnish with fresh parsley or thyme and serve with slices of French baguette or another toasted crusty bread.