Wash and dry all produce. Adjust rack to middle position and preheat oven to 425 degrees. Cut tomato into quarters. Place tomato and garlic on a large piece of aluminum foil. Season with salt, pepper, and 1 tsp sugar. Wrap foil around ingredients, sealing edges to create a pouch. Roast in oven until very soft, 20-25 minutes.

Meanwhile, slice potatoes into 1/4-inch-thick rounds. Roughly chop chives. Whisk together milk, half the truffle zest, 1/2 cup water, and flour in a medium bowl. Heat a medium pan over medium-high heat (use an ovenproof pan if you have one). Add potatoes and half the chives, then season generously with salt and pepper.

Pour milk mixture over potatoes in pan, then cover. Bring to a simmer and let bubble for 5 minutes. (TIP: Transfer everything to a small baking dish after simmering if your pan is not ovenproof.) Uncover and sprinkle with Parmesan. Transfer pan to oven and bake until potatoes are very tender, about 25 minutes.

After potatoes have baked for 10 minutes, heat a large drizzle of olive oil in a large pan over medium-high heat. Season steak all over with rosemary, salt, and pepper. Add to pan and cook until browned on bottom, 3-4 minutes. Flip and continue cooking to desired doneness, 2-4 minutes more. Remove from pan and set aside to rest, covered, on a plate. Heat a drizzle of olive oil in same pan over medium-high heat.

Add spinach to same pan and season with salt and pepper. Cook, tossing, until wilted, 1-2 minutes. Remove from pan and divide between plates. Remove tomato and garlic from foil and add to same pan over medium-high heat. Pour in stock concentrate and 1/4 cup water. Mash tomato and garlic with a potato masher or wooden spoon until a chunky sauce forms.

Simmer sauce in pan until slightly thickened, 1 minute. Season with salt and pepper. Stir in any juices released by steak. Cut steak into 1/2-inch slices. Divide between plates with spinach and spoon sauce over. Add potatoes to plates. Sprinkle with remaining truffle zest. Garnish with remaining chives.