1. Slit dates lengthwise down 1 side and remove pits. Squeeze dates from ends to open slits, and fill equally with blue cheese. Press dates to close around cheese. Wrap a piece of bacon around each date, securing with a toothpick. Place dates in a 10- by 15-inch pan.
2. Bake in a 450° oven until bacon is well browned, about 10 minutes (about 6 minutes in a convection oven); after about 4 minutes, use a wide spatula to ease dates loose from pan and turn over.
3. Meanwhile, in a bowl, combine oil and vinegar. Add spinach and mix gently, seasoning to taste with salt and pepper.
4. Spoon spinach mixture onto plates. Remove toothpicks from dates and set an equal number on each salad.