Serve it tonight: Combine first 2 ingredients. Form into 12 patties. Place 1 Tbsp. blue cheese in the center of 6 patties. Top each with another patty and pinch edges to seal in cheese. Pan fry or grill over medium heat until internal temperature reaches at least 160°. Salt and pepper to taste.
Meanwhile, add onions and oil to a large skillet. Sautè onions over medium heat for 1-2 minutes. Layer onion, burger, spinach and remaining blue cheese on buns.
Freeze it for later: Assembly patties as directed above. Place between sheets of waxed paper and place in gallon resealable freezer bag. Seal well, label and freeze. Thaw completely. Prepare as directed above.
Tip: Substitute ground turkey, if desired.
Serve with fresh vegetable sticks.