Prepare vegetables: Peel and cut potatoes into even-sized chunks. Peel and slice carrots and celery thinly. Chop onion.
Preheat Oven: Preheat oven to 450.
Cook vegetables: In a saucepan, boil potatoes. Cook until potatoes are soft enough to mash – easily pierced with a knife or fork (15 to 20 minutes). In a large pot, sauté onion in oil until softened. Add carrots and celery, and cook until tender. Add garlic, thyme, and oregano at the end and toast, being careful not to burn.
Brown ground beef: Add ground beef to the pot and brown. Break up any large clumps. Cook until no longer pink (drain grease if needed).
Add seasonings: Stir in salt and pepper.
Thicken sauce: Add water and simmer for a minute. Then add flour, and stir until combined. Cook for 1 minute. Now add tomato paste, sriracha, browning and seasoning sauce, Worcestershire sauce, and salt and pepper
Simmer: Gradually add chicken broth, stirring constantly, until thickened.
Add tomato paste: Stir in tomato paste, hot sauce, and Worcestershire sauce. Cook gently for about 5 minutes. Remove from heat.
Make mashed potatoes: Drain and mash cooked potatoes with butter, milk, salt, and pepper.
Assemble shepherd's pie: Spoon the meat mixture into a baking dish. Top with mashed potatoes and sprinkle with shredded cheddar cheese.
Bake: Bake in a preheated oven at 450°F (190°C) for 25 minutes, or until golden brown and bubbly. Allow to rest for about 5-10 minutes for best results!