1. Heat the oil and butter in a cast-iron, or other heavy bottomed skillet, until hot, but not smoking. Add the garlic and stir for 30 seconds.
2. Using paper towels, gently pat dry the shrimp, scallops and capers then add them to the pan. Cook until the shrimp and scallops are cooked through and starting to brown, 5-6 minutes. Remove from the pan with a slotted spoon.
3. Keep the pan on medium heat and stir in the spinach. Cook 1-2 minutes, or until the spinach has wilted. Add the vinegar and toss to coat.
4. Immediately divide the spinach between four plates. Top with the shrimp and scallops and serve.