The brassicas here include baby cauliflower, cute 2- to 4-inch heads that come in vibrant colors like green, orange and purple as well as the usual ivory. Slideshow: More Steak Recipes
Preheat the oven to 425°. On a large rimmed baking sheet, toss the brassicas and thyme with 1/4 cup of the olive oil and season generously with salt and pepper. Spread in a layer and scatter 2 tablespoons of the butter on top. Roast for 20 to 25 minutes, until golden and tender.
Meanwhile, in a large cast-iron skillet, heat the remaining 1 tablespoon of olive oil. Season the steaks all over with salt and pepper. Add to the skillet and cook over moderately high heat, turning often, until an instant-read thermometer inserted in the thickest part of each one registers 120°, 12 to 15 minutes. Transfer to a carving board and let rest for 10 minutes. Do not wipe out the skillet.
In the same skillet, cook the shallots and garlic over moderate heat, stirring, until softened, 2 minutes. Add the wine and cook, scraping up any browned bits, until almost evaporated, 3 minutes. Add the stock and cook until slightly reduced, 2 minutes. Remove from the heat and gradually whisk in the remaining 5 tablespoons of butter until emulsified; season with salt and pepper. Carve the steaks against the grain. Serve with the brassicas and sauce.