Preheat oven to 375°F.
Bring steaks to room temp and drizzle liberally with olive oil, rub with crushed garlic and stemmed rosemary leaves, and season liberally with Kosher salt, black pepper. Let stand, covered, for 1 hour.
Heat a cast-iron skillet over medium-high heat. Add steaks, keeping garlic cloves atop steaks with tongs. Cook to crispy brown on each side (about 2 minutes per side) then transfer to oven to finish cooking for about 5 minutes. Let rest on a carving board before slicing for 10 minutes. Slice against the grain, fairly thin (about 1/4 inch).
Meanwhile, heat a turn of the pan olive oil in a medium skillet over medium-high heat. Melt butter into oil and when it foams, add the mushrooms. Brown until meaty, fragrant and deep golden; add thyme, sage, salt and pepper, and toss to combine. Add brandy and swirl a minute; add Worcestershire, toss and remove skillet from heat.
To the pan you cooked steaks in, add a drizzle of oil and wilt in greens. Dress with salt, pepper and lemon juice.
Top sliced streak with mushrooms and serve greens alongside.