Firstly, To the marinade: In a large bowl or a ziplock bag add the garlic cloves, tomato paste, sugar, vinegar, beer salt, pepper and mix well.
With a sharp knife make superficial cuts into the beef’s fat. This will help to absorb the marinade better and will melt during roasting, softening the meat. Put the beef and the thyme branches into the marinade, cover the bowl with cling film or seal the ziplock bag and leave it into the fridge for at least 2 hours or up to 24 hours for a more flavored meet.
Preheat the over to 200º C, remove the flank steak from the marinade, and transfer it to a sieve over a bowl. Leave a few minutes the excess of the marinade. Reserve the marinade.
Cut the onions in half lengthwise keeping the roots and cut 1 cm slices. Cut the bell pepper in half lengthwise, remove the seeds, and cut slices of 0.5 cm.
In a roaster, put the onion slices to cover the bottom and accommodate the flank steak over it. Season the onions with salt and black pepper to taste. Gently laid the beef over the onions and add 1 tablespoon of olive oil, making sure to spread all over with your hands.
Add the bell peppers slices, garlic, and thyme branches from the marinade on the roaster corners. Season them with salt and black pepper to taste and 1 tablespoon of olive oil. Then fill the roaster with the marinade, but don’t pour over the beef.
Roast for around 35 minutes at 200º C until golden the flank steak and the veggies soften. Use a meat thermometer to check the cooking temperature if you want. For a medium-rare, the temperature inside must be at 57 º C. For well-done it’s 65 º C.
Remove the roaster from the oven and transfer the meat to a chopping board or large plate to rest and accommodate the internal juices for 10 minutes before slice. Transfer the liquids left on the roaster to a sauceboat or a bowl and the roasted veggies to another bowl.
To slice the flank steak properly always cut against the grain. The lines in the meat are the direction you want to find and cut the opposite of those lines. In conclusion, This will enhance the flavor of the meat as well as keep it tender.