Preheat oven to 425 degrees.
Rub steak with 1 tablespoon olive oil. Season beef with salt and pepper. In a large casserole on the stovetop, sear beef on both sides. To a heavy roasting pan add onions and 2 tablespoons olive oil and roast in oven for 20 minutes, to begin caramelizing onions. Place charred roast on top of onions and roast until internal temperature is 135 degrees (for medium rare), or higher, about 45 minutes. In final 15 minutes (internal temperature of beef should be approximately 120 degrees), mix garlic, parsley, extra virgin olive oil, and lemon juice. Brush mix on steak as roasting marinade and brush on mushroom caps. Add mushroom caps to roasting pan while the beef cooks for final 15 minutes.
Remove roast, place on carving board, and allow to rest. Place onions and mushroom caps on platter.
Pour excess fat from roasting pan, leaving 1 tablespoon. Add flour and cook on stovetop for 2 minutes. Add wine and reduce by half. Add broth and cook for 2 minutes.
Carve roast. Serve with onions, mushrooms and gravy.