Season steak cubes with salt, pepper, garlic powder, and avocado oil; refrigerate.
Boil cubed potatoes in salted water until tender.
Mash boiled potatoes with 2 tablespoons butter and warm whole milk; season with salt and pepper.
Caramelize sliced onions in a skillet until golden brown and set aside.
In a saucepan, combine beef broth, Worcestershire sauce, garlic paste, lemon zest, lemon juice, rosemary, thyme, and Dijon mustard; simmer and strain.
Prepare gravy by making a roux with 3 tablespoons butter and flour; gradually whisk in the strained broth until thickened.
Sear seasoned steak bites in a hot skillet until browned to desired doneness.
Serve steak bites over mashed potatoes, drizzle with gravy, and top with caramelized onions and fresh chives if desired.