Roast the chillies in a pan and keep them aside
Add a little oil and add the onions, garlic, peppercorns & cloves
Remove when they have some color
Off the tomatoes and allow to char and remove
Add the chillies back in, cover with water and allow to come to the boil and cook for 30 mins
In a food processor, add the chillies, onion, garlic, peppercorns, cloves & tomatoes
Add the Oregano and salt and some of the chilli water and blend to a puree
You may strain the sauce if you wish
Marinate the shrimp with salt, pepper and the chilli powder
Add some oil to the wok and just cook the shrimp on both sides
Add the sauce and cook for a few minutes before serving
Garnish with a pinch of Mexican chilli powder