Season the steak cubes with SPG seasoning and avocado oil, toss to coat, and refrigerate while preparing the other components.
Peel and cube the potatoes, then boil in salted water for 15-20 minutes, or until fork-tender. Drain and mash using a potato ricer, then mix with 2 tablespoons butter, warm milk, and season with salt and pepper. Stir in 2 additional tablespoons of butter and keep warm.
To caramelize onions, slice 2 large onions thinly and cook them in 2 tablespoons butter over medium heat for 20-30 minutes, stirring occasionally, until golden brown.
Prepare the au jus by simmering beef broth, Worcestershire sauce, garlic paste, lemon zest, lemon juice, rosemary, thyme, Dijon mustard, and butter for 15 minutes. Strain and set aside.
Make the gravy by melting butter in a pan, whisking in flour to create a roux, and slowly adding the au jus while whisking. Simmer until thickened, about 4-5 minutes.
Heat a cast-iron skillet over medium-high heat and sear the steak bites for 2-3 minutes per side until browned and cooked to your preference.
Assemble the dish with mashed potatoes as the base, steak bites on top, and a drizzle of gravy. Garnish with caramelized onions and chopped chives. Serve warm and enjoy!