Heat grill to medium-high heat.
Brush onions and tomatoes with oil. Grill 2 min., turning tomatoes after 1 min. Remove tomatoes from grill. Grill onions 6 min., turning after 3 min. Cool.
Meanwhile, mix meat and oregano; shape into 8 (1/2-inch-thick) patties. Grill 3 to 5 min. on each side or until done (160ºF), topping with Singles pieces and adding buns to the grill to toast for the last 2 min.
Chop onions and tomatoes; place in medium bowl. Add vinegar and lemon juice; mix lightly. Spoon onto bottom halves of buns; cover with spinach, burgers and tops of buns.