Pat the beef cubes dry with paper towels. Season evenly on all sides with coarse sea salt, black pepper, garlic powder, and onion powder.

Heat 2 tablespoons of oil in a large skillet over medium-high heat. Sear beef cubes in batches for 2 to 3 minutes per side until deeply browned with a rich crust. Transfer seared beef into the slow cooker crock.

Reduce skillet heat to medium and melt 1 tablespoon of butter. Add sliced mushrooms and diced onions; sauté for 4 to 5 minutes until tender and caramelized. Add minced garlic and sauté for 1 minute until fragrant.

Pour 1/2 cup of beef broth into the skillet, scraping up all browned bits from the pan bottom. Pour the mushroom mixture and skillet juices into the slow cooker over the beef.

Add the remaining 2 cups of beef broth, Worcestershire sauce, Dijon mustard, fresh thyme, and rosemary. Stir gently to combine.

Cover and cook on LOW for 7 to 8 hours (or on HIGH for 4 to 5 hours) until the beef tips are extraordinarily fork-tender.

In a small bowl, whisk 3 tablespoons of cornstarch with 3 tablespoons of cold water until smooth. Pour into the slow cooker, stir gently, and cook on HIGH for 20 to 30 minutes until the gravy is thick and glossy.

Discard thyme and rosemary stems. Ladle steaming beef tips and generous scoops of mushroom gravy over fluffy garlic mashed potatoes. Garnish with fresh minced parsley and serve immediately.