Preheat oven to 220°C (200°C fan-forced). Preheat 2 rimmed baking trays in oven. In a medium bowl, whisk 3½ tbs oil, vinegar, shallots and thyme. Season with salt and pepper.
In a separate medium bowl, toss the broccoli and mushrooms with 2 tbs oil. Season with salt and pepper. Arrange broccoli on 1 hot baking tray. Arrange mushrooms, cut-side down, on the remaining hot tray. Roast vegetables, turning the broccoli halfway through cooking, for 25 mins or until broccoli is browned and lightly crisp and the mushrooms are browned and tender.
Pour the stock over mushrooms and scrape the base of the tray to loosen any browned bits. Transfer to a large heatproof bowl. Add broccoli and half the vinegar mixture. Season with salt and pepper. Toss to combine.
Meanwhile, place a wire rack on a rimmed baking tray. Coat the steaks with 2 tsp oil and season with salt and pepper. Heat a large cast-iron skillet or heavy-based frying pan over medium-high heat. Cook the steaks for 2 mins each side for medium-rare doneness or until cooked to your liking. Transfer to a clean wire rack. Set aside for 5 mins to rest.
Slice the steaks against the grain. Arrange on serving plates with the roasted mushrooms and broccoli. Serve with remaining vinegar mixture.