Combine meat with garlic, vinegar and 1/4 cup of the olive oil in a large resealable plastic bag. Marinate in refrigerator for 3 to 6 hours.
Heat oven to 400°. Remove meat from refrigerator so it can come to room temperature.
Cut onions into quarters and halve shallots. On a large rimmed baking sheet, toss together onions, shallots and remaining 1 tbsp olive oil. Season with salt and pepper and roast at 400° until tender, 40 to 50 minutes.
After veggies have been roasting for 20 minutes, pour off and discard marinade from meat. Pat meat dry and season it all over with salt and pepper. Heat a large ovenproof skillet over medium-high heat and sear meat until nicely browned, about 5 minutes per side. Transfer skillet to oven and cook alongside vegetables until an instant-read thermometer inserted into meat reads 130° for mediumrare, 12 to 18 minutes. Remove meat and veggies from oven. Let meat rest for 5 minutes before slicing.