Finely chop (or crumble) the chorizo sausage - you might need to peel the skin off first. Place the chorizo crumb in a cold non stick pan and set over a medium heat. Allow to cook until the chorizo has released its oils and crisped a little. You may n need to add some oil to help it along. Remove from heat and allow too cool.
Remove the oysters from their shells. Wash the shells in hot water, then pat dry. Replace the oysters and cover with a damp cloth in the refrigerator.
Put the tomato, shallots, chilli and coriander in a bowl and mix together well. Stir in the lime juice, then season to taste with salt and pepper.
Put a teaspoon or so of the chorizo crumb on each oyster and top with a teaspoon of salsa.