Start preparing the marinade. In a large bowl with lid or ziplock bag add the beer, garlic, sugar, vinegar, tomato paste, salt, black pepper, close and mix.
With a sharp knife, cut superficially the meat’s fat.
Arrange the beef inside the bowl or ziplock so the marinate covers all sides of your beef. Leave in the fridge from 2 hours up to 24 hours marinating.
When ready to cook, preheat your oven at 392 º F / 200ºC.
Remove the flank steak from the marinade and transfer to a sieve over a bowl to drip extra marinade. Reserve the marinade.
Arrange the sliced vegetables in a roaster, start with the onions covering the whole surface of roaster. Season with salt and pepper to taste.
Arrange the beer marinated flank steak over the onions, add the remaining veggies on the roaster corners and pour the marinade, but don’t pour over the meat.
Roast for 35 minutes at 392ºF / 200ºC until meat and vegetables are golden brown.
Check the internal temperature of beef with a meat thermometer. For a medium rate, it should be at least 135ºF / 57ºC. For well done, cook until it reaches 150ºF / 65ºC.
Remove the roaster from the oven, transfer the meat to a chopping board or plate and leave to rest for 10 minutes before slicing.
Pour the liquids from the roaster in a sauceboat and roasted veggies to another bowl.
After resting, cut the meat against the grain for the most tender cut of beef and serve.