Heat butter over medium heat. Add onions and season with salt and pepper. Cook to very sweet and caramel in color, 25-30 minutes. Add flour and stir 1 minute; add beer, Worcestershire and stock. Reduce heat and gently thicken gravy, 10-15 minutes more.
While onions cook, peel and chop up potatoes. Place them in a large saucepot, cover with water and bring to a boil. Salt water and cook potatoes to tender. Drain and return them to the hot pot. Mash with milk or half-n-half, mustard, salt, pepper and a little nutmeg, to taste. Mash spinach into potatoes to wilt.
While gravy thickens, cook steaks 8-10 minutes on a hot griddle or in a cast-iron skillet over medium-high heat.