Preheat oven to 400 F. Remove outer peeling on garlic head, but to not break apart cloves. Cut top of garlic head to expose cloves. Drizzle olive oil over the top. Wrap head of garlic in foil or place in garlic roaster, and roast in preheated oven for 30 to 35 minutes. Remove roasted cloves and mash. Set aside.
In a large bowl, whisk together vinegar, oregano, salt until salt is dissolved. Add sliced onion and toss to coat. Let stand until ready to use.
In a small sauce pan, simmer oil, bay leaves, garlic, chilies, cumin, mustard and peppercorns in for 10 minutes over medium heat, then let stand until ready to use.
In a medium sauce pan, bring salted water to a boil over medium high heat. Add shrimp and let boil 5 minutes or until shrimp are cooked. Drain and pat shrimp dry. Add shrimp to onion mixture and toss to coat. Pour oil and spices over the top and stir to combine.
Let cool, then cover with plastic wrap or lid and chill in the refrigerator 2 to 4 hours until ready to serve. Shrimp may be served cold or at room temperature.