Season steak with 3/4 teaspoons each salt and pepper. Heat a lightly oiled heavy skillet on medium-high heat. Cook steak 5-6 minutes per side for medium (or preferred doneness). Rest for 10 minutes then slice thinly.
Meanwhile, peel and sement grapefruit and orange over a bowl to capture any juice. Whisk 2 tablespoons of the collected juice with the vinegar, garlic, ginger paste, mustard, honey, and remaining salt and pepper until combined. Drizzle in oil while continuing to whisk.
Toss spinach fruit and warm steak together with the dressing and garnish with pomegranate seeds. Serve with rusty bread, if desired.