Wash and dry all produce. Peel potatoes, then cut into 1/2-inch cubes. Place cauliflower and potatoes in a large pot with a pinch of salt and enough water to cover by 1 inch. Halve lemon. Squeeze juice from one half into pot. Bring to a boil and cook until potatoes and cauliflower are tender, about 15 minutes. Drain and return to pot.

Meanwhile, heat 1 TBSP oil in a large pan over medium-high heat. Add entire bunch of rosemary. Cook, tossing, until crispy and fragrant, 2-3 minutes. Remove from pan and discard, keeping as much oil in pan as possible. Pat steak dry with a paper towel. Season all over with salt and pepper. Add to pan and cook to desired doneness, 3-6 minutes per side.

Remove steak from pan and let rest on a plate, covering loosely with foil to keep warm. Carefully pour oil from pan into a small bowl and set aside. After returning potatoes and cauliflower to pot, mash with potato masher or fork until mostly smooth. Season to taste with salt and pepper. Cover and keep warm on stove over low heat. Thinly slice garlic.

Using a paper towel, carefully wipe clean pan used for steak, then heat a drizzle of oil in it over medium-high heat. Add garlic and cook until fragrant, about 30 seconds. Toss in spinach. Cook until tender and wilted, 2-3 minutes. (TIP: Add a splash of water if spinach won't wilt.) Season with salt, pepper, and a squeeze of lemon. Remove from pan and set aside.

Add 1/2 cup water and beef demiglace to pan. Bring to a boil and let bubble until reduced by half, 2-3 minutes. Remove from heat. Season to taste with salt and pepper. Stir in any juices from steak that have collected on plate.

Slice steak against the grain. Divide mash and spinach between plates, then top mash with steak. Drizzle with sauce and reserved oil.