Following the bag instructions, place the shrimp in a strainer under cold running water until thawed. Drain well and put back in the fridge until it is time to cook them.
Warm up oil in a big pan over medium-high heat. Sauté onion for a couple of minutes, add garlic and serrano chiles. When the onion is soft and translucent add tomatoes and cook for two minutes more.
Add tomato paste, thyme, bay leaf, beer and salt. Bring to boil the boil, then reduce to medium heat, cover and simmer for 25 minutes.
Submerge the shrimp in the sauce. Cook 4-6 minutes stirring now and then to make sure they cook on both sides. Please don't overcook them. The shrimp are ready when they start to curl and turn pink
Garnish with green onions if you like and serve with white rice and warm tortillas.