Preheat oven to 400 F. Bring a large pot of salted water to a boil, then add potatoes, cooking for about 12-15 minutes until fork tender. Drain, then add in butter, sour cream, salt & pepper. Mash with a potato masher until roughly combined.
Meanwhile, heat a large skillet over medium-high heat. Add ground beef, breaking up with a spoon, then add onions, cooking for 5 minutes. Add in garlic, salt and pepper and thyme, cooking another 30 seconds until fragrant.
Stir in flour until beef mixture is coated, then add in beef broth, Worcestershire and frozen veggies. Cook another 2 minutes until mixture thickens then remove from heat.
Grease a 9 x 13 casserole dish, then add beef mixture. Top with mashed potatoes, spreading across evenly. Top with cheese, then bake for 20 minutes.
Remove from the oven, top with chives if desired, then serve and enjoy!