1. Soak the skewers in cool water for 30 minutes.
2. Blend lime juice, garlic, achiote seeds, allspice berries, oregano, salt and peppercorns in a KitchenAid Blender until smooth.
3. Gradually add olive oil to the mixture as the blender is running to create an emulsion.
4. Assemble the skewers with 3 shrimp, 3 pineapple wedges, and 2 onion pieces and transfer to a shallow pan.
5. Add three sprigs of fresh oregano to the pan of skewers once they are finished being assembled. Pour the achiote paste over the skewers evenly and marinate for 1-3 hours.
6. Prepare a charcoal grill to high heat.
7. Place the skewers on the grill and cook about 3-4 minutes a side.
8. Serve with a squeeze of lemon.