Wash and dry all produce. Preheat oven to 425 degrees. Halve, peel, and thinly slice onion. Strip rosemary leaves from stems and finely chop until you have 1 TBSP. Discard stems. Halve potatoes lengthwise, then cut into 1-inch-thick wedges (like steak fries).

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

Heat a large drizzle of oil in a medium pan on medium-high heat. Add onion and cabbage and cook until softened, about 5 minutes, tossing. Add mustard seeds and cook until they start to pop. Stir in white wine vinegar, sugar, and a pinch of salt. Reduce heat to low, cover, and cook until cabbage is 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 sear until browned and cooked to desired doneness, 4-7 minutes per side. Remove and set aside to rest.

Add 1/2 TBSP balsamic vinegar, 11/2 TBSP soy sauce, cranberry jam, and 3 TBSP water to same pan over medium heat. Stir to combine. Let simmer until reduced to a syrupy consistency, about 2-3 minutes. Remove from heat.

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