Wash and dry all produce. Adjust rack to middle position and preheat oven to 425 degrees. Halve, peel, and thinly slice onion. Pick and finely chop enough rosemary leaves from to give you 1 TBSP. Halve potatoes lengthwise, then cut into 1-inch-thick wedges (like steak fries).

Toss potatoes on a baking sheet with chopped rosemary and a large drizzle of olive oil. Season with salt and pepper. Roast in oven until lightly browned and crisp, 30-35 minutes.

Heat a large drizzle of oil in a medium pan over medium-high heat. Add onion and cabbage. Cook, tossing, until softened, about 5 minutes. Stir in mustard seeds and cook until they start to pop, about 1 minute. Stir in white wine vinegar, 1 tsp sugar, and a pinch of salt. Reduce heat to low, cover, and cook until tender, about 10 minutes.

While cabbage cooks, heat a drizzle of oil in another medium pan over medium-high heat. Pat steak dry with a paper towel, then season all over with salt and pepper. Add to pan and cook until it reaches desired doneness, 4-7 minutes per side. Remove from pan and set aside to rest.

Add 1/2 TBSP balsamic vinegar, 11/2 TBSP soy sauce, cranberry jam, and 3 TBSP water to pan used for steak over medium heat (we sent more vinegar and soy sauce). Stir to combine. Let simmer until reduced to a syrupy consistency, 2-3 minutes. Remove from heat.

Thinly slice steak against the grain. Divide steak, potatoes, and cabbage between plates. Drizzle sauce over steak and serve.