Preheat the oven to 400 degrees Fahrenheit. Core, seed, and thinly slice the bell pepper. Toss on a sheet pan in olive oil, salt, and pepper. Peel and smash the garlic cloves with a knife. Pull the leaves off one of the sprigs of rosemary. Add the garlic and rosemary to a foil pack and cover with a generous amount of olive oil. Roast the bell pepper and the foil pack for 10 minutes.
Pat any excess juice off the steaks with a paper towel. Season both sides with diamond kosher salt and freshly ground black pepper. Heat a large drizzle of olive oil in a cast iron skillet over medium heat.
Sear each side of the steak for 3-4 minutes until brown and nicely seared but not fully cooked. Add the steaks to the sheet pan with a pat of butter and sprig of rosemary on top of each one to continue roasting for 6-8 minutes or until desired temperature is reached.
Slice the bread on the diagonal. Lay several pieces flat on the sheet pan and drizzle with a generous amount of olive oil. Bake for 5 minutes.
Mince the roasted garlic and rosemary. Add to a small bowl with 2 tbsp white wine vinegar and 1 tbsp olive oil. Whisk to combine then season with salt and pepper.
In a large bowl, toss together the bell pepper and greens then drizzle with the vinaigrette, tossing until all the greens are evenly coated.
Once the steaks have reached the desired temperature, allow to rest for 5 minutes, then cut into thin slices. Plate the steak atop a large portion of the dressed greens. Top with feta cheese and serve with the sliced bread.