Zest half of lemon and set lemon and zest aside. Preheat oven to 450°F.
In a bowl, toss together fingerlings, olive oil, rosemary, salt and pepper. Place in ovenproof casserole dish and roast until browned and softened (25-30 minutes), stirring once or twice during roasting.
While potatoes are roasting, season porterhouse steak with salt and pepper. Sear steak on a grill pan over high heat, 2 minutes per side. Reduce oven temperature to 350°F. Add asparagus and steak to pan and place in oven until asparagus softens and steak is cooked to desired doneness, approximately 5 minutes for medium rare. Remove steak from pan and allow to rest on cutting board, tented with foil.
Slice lemon in half; juice one half over roasted potatoes. Add asparagus, lemon zest and parsley; stir to combine and divide among two plates. Cut meat from the “t-bone” of the porterhouse. Slice if desired and serve tenderloin or strip steak cuts according to preference.