In a small saucepan, heat 1 cup vinegar on medium. Bring to a gentle simmer and cook until syrupy and reduced by about three-quarters. Remove from heat and set aside.
Meanwhile, prepare marinade: In a medium bowl, whisk shallot, garlic, basil, red pepper flakes and remaining 1 tbsp vinegar until well blended. Slowly whisk in oil until blended. Season with salt and black pepper. Remove 2 tbsp marinade, transfer to a small bowl and set aside; add prawns to remaining marinade and toss to coat.
Heat an outdoor grill on medium-high. Pinch rosemary sprigs one-quarter of the way up from bottom stem and pull down with your fingers to remove bottom leaves. Carefully thread prawns onto bare ends of rosemary sprigs. Season with salt and black pepper and add to grill. Cook for 2 minutes per side, until just cooked through. Remove from grill and let rest for 1 minute.
In a large bowl, toss spinach and tomatoes with reserved 2 tbsp marinade. Season with salt and black pepper. Serve rosemary-prawn skewers over salad and drizzle with reduced vinegar.
Serving Size: 2 grilled prawn skewers and 1 cup salad