Preheat the oven to 425 degrees .
In a deep skillet, bring a few inches of water to a boil. Salt the water, add the broccoli rabe and cook until just tender, 2 to 3 minutes. Drain and transfer to a kitchen towel to dry. Transfer to a foil-lined baking sheet, season with salt and pepper and toss with 2 tbsp. EVOO and the crushed garlic and crushed red pepper. Spread out in an even layer and roast until crispy at the edges, 18 to 20 minutes.
Meanwhile, in a large saucepan, cover the potatoes with water. Bring to a boil and cook until just tender, 8 to 10 minutes. Drain, return to the pan and mash with the milk and cheese; season and cover.
In a small bowl, mix the butter, rosemary and chopped garlic.
Heat a cast-iron skillet or griddle over medium-high. Coat the steaks with 1 tbsp. EVOO and season liberally with salt and coarsely ground pepper. Using a paper towel, rub the hot skillet with the remaining 1 tbsp. EVOO. Add the steaks and cook, turning once, about 8 minutes for medium-rare.
Divide the steaks among plates. Top each with about 1 1/2 tbsp. rosemary-garlic butter and sprinkle with flaky sea salt. Serve with the broccoli rabe and mashed potatoes.