"Magic" Spinach Brownies

Here's the recipe for the dish I brought to the BBQ on Monday.

“Magic” Spinach Brownies

(1)
1 tsp salt
1 cup flour
1 lb. grated cheddar or mozzarella cheese
1 tsp baking powder

(2)
2 eggs
1 cup milk
1/3 cup margarine (melted)
1/2 - 3/4 cup finely chopped onion
1 pkg. frozen spinach thawed & drained (or fresh spinach equivalent to 1 pkg. frozen)

Mix (1) and (2) and put in 9” x 13” pan. Bake at 350˚F for 40-45 minutes.

Notes:
a) “Spinach Brownies” recipe is from Jacquie courtesy of Sheila.
b) My version of spinach brownies turned out as if by “magic,” despite the following two shortcomings: I guesstimated the amount of fresh spinach needed to equal a frozen package; and I had to combine Monterey Jack, cheddar, one slice of Swiss, and pinches of Parmesan to make what I hope was close to 1 lb. of cheese. (I stopped short of incorporating the Kraft Singles grilled cheese slices.)
c) The ingredients should make it obvious, but just in case, please don't mistake these for a "dessert" brownie; no chocolate in this one!